Thank you to everyone that replied. I honestly don't even know what to make of this situation anymore. When I asked him why he didn't tell me everything that was going on with his kid before, he just tells me that his kid is perfect in his eye. It really boils my blood when he says that because it feels like a total cop out. I agree that SS shouldn't live with grandparents forever because he isn't their kid and not their responsibility. I also don't feel this is my responsibility. If he was my birth child or I was in the picture sooner, I would've had him start a program when he was 2-3 years. Why should I pay the price for their delayed treatment? 

DH is definitely in denial. He is happy to pass the buck and not deal with anything. I am worried about what will happen in the future because I don't want to be the one to take the buck. I don't even know how to talk to him about it without coming off as if I don't like the child or don't want him around (which is probably true, but I don't want that to come out during a sensitive discussion). DH is fine to just let things go and happen as they come, but I don't know if I want the kid around forever. I don't know how I can make the decision at this point with so many uncertainties. When I ask questions, I can't get an answer because DH has no answer. It's as if he hasn't thought much about it either. I don't really want to get a divorce because I do love my DH and I don't really like to use divorce as an option. However, I don't like living with these uncertainties and always wondering how bad it's going to get and how much I will have to handle and how much I will have to "be patient" and give in on my end. I am trying to figure how I can make this work for me.

Also, without going into too much detail, there is a huge cultural element present here as well. I don't want to sound like I'm making excuses for DH, but he and his family are from another country where autism is highly frowned upon. My MIL doesn't even like to hear the word autism. I am trying to be understanding to the cultural differences, but with all this I feel like disengaging and letting them just do everything with the kid. I can help out with basics like making meals and pickups/dropoffs, but other than that I think disengaging might be a good idea at this point.

I have told him this a couple of times. He didn't become hostile. I would say he was defensive over his current actions and tried to reassure me, but I wasn't very convinced because I can't say I believe his reasons for not doing his best right now. I have questioned why he didn't get SS help earlier with no specific answer. He has been uninvolved, but I don't know why. Sometimes I think the revelation comes down to his wording and what I can tell from conversations with him is that he didn't actually want SS, but took him out of responsibility. If we have a baby together, he actually wants the baby, perhaps because the relationship is loving and stable or maybe he has someone to share the work with (not sure). He's careful with his wording sometimes, but the more we talk the more I hear, which in combination with his lack of interest in spending time with SS makes more sense that this is how he feels rather than "my kid is perfect in my eyes." It doesn't matter at the end of the day, though, because SS will have to live with us at some point and you are correct. When I realized he had lied big time, I was very mad. When I brought it up, I expected an explanation, but I never got any. It was just like well you've seen how he is now and I was just expected to deal with it. No plan or compromise or anything. Even though he lied and I was angry about it, what could I do except leave? DH and I are actually pretty happy together when it's the 2 of us. If SS wasn't in the picture, I feel like everything would be fine, which leads me to feeling resentful of SS and that's not really what I want. 

I think I just need to make my own rules and set my own expectations about what I will/won't do. I am not sure how to effectively communicate this without an argument, though. Last time I said I didn't feel comfortable watching SS for several hours after school by myself until DH gets home, DH was very angry about it. He thinks since I would do this for my own child, I should do it for his and it's only a few hours so it shouldn't be a big deal, but it is a big deal because he isn't my child and I don't want it to take away time from my own child (if we have a child). I am expected to act like the kid's BM and do everything for him, but I don't want to.

I tried to have a talk with DH last night again about my worries after getting opinions on here. The talk didn't amount to much as usual. He said the same thing as he usually says: he believes that whatever I do for my child, I should do for his. I tried to explain to him that I am a stepmom, not a bio mom. I didn't have the child myself so the responsibility isn't mine. He thinks that since we are married I should share the responsibility with him. I reiterated I don't mind helping with things that I am COMFORTABLE helping with, but if I choose to help that should be up to me. He said he thought I was different from other girls and he waited so long to date someone because he was looking for someone that could love SS like their own kid. When we first started talking, he told me girls here don't like kids. Of course I like kids. What he didn't say is his son has serious challenges and special needs and no one wanted to deal with the baggage. Not that they don't like kids. I don't even know how to respond to this.When I try to question the reason of him bringing me into the relationship and allude to him wanting to use me for childcare, he says that he and SS were fine before. He says he had a lot more expendable income and could have taken a job with less pay if need be. I'm not sure how he can say this considering he's working the same job now that he was before he met me and he is actually in debt. His child was definitely not doing well and no improvement or even action towards improvement. Me, however, yes I was definitely better. I had a decent paying job, more independence, no debt and less obligations. I don't know why he even brings this up because I don't.

I also tried to explain again that being a stepmom will mean that I will naturally have a stronger tie to my own child. I would not choose his child over mine ever. That's just nature and ingrained in us as human beings. It's not something I can change and I already know that is how I would feel. If I had to choose between the 2, I would choose my own child. I know that and I don't like being forced into the position of having to make a choice. He said that I said before (a long time ago before I had even met the child) that I wanted to be like a mom to SS. I did say that and at the time I thought it was possible and also under the impression the child had been tested  by a doctor and was not autistic. I was not made aware of his problems and issues at all. DH wasn't really angry, but he seemed disappointed. He was saying how he is lucky he always has his family to back him up and this is why his family doesn't really trust me to be alone with SS and he doesn't fully trust me either. I was like well then why are we even talking about this? If everyone feels that way why would he ever live with us and why would I ever have to experience the scenarios I've laid out. It makes absolutely no sense to me.

DH is in denial about how bad his child's behavior is. He feels like me watching the kid for a few hours alone every day is no big deal (mind you, he said he didn't fully trust me alone with SS) because his kid doesn't get in to trouble. I had to remind him how the kid often tries to escape grandparent's house. DH denied it, saying his son isn't tall enough and threatened to call his mom to confirm. I was like that's ridiculous as this is a discussion between us and your mom has told you several times. I told him the last time she found him in the garage under the car. I told him when I went over to the house yesterday (I often go to see SS by myself when DH is working. I actually spend more time with SS than DH right now), SS had put a bottle of Clorox in his mouth. What if he had opened it and drank it? I told him that he doesn't know what his mom goes through on a daily basis. He just said he is thankful for his mom's help, but he is living in a fantasy world. He has been spoiled by his family with childcare and doesn't know the reality. I am not willing to devote 100% of my time to watching a child that could accidentally end up in the ER nearly everyday and I am not willing to pickup the burden of childcare his mom endures everyday, either.

In the end, we agreed to speak to a counselor to get a second opinion and try to arrive at some sort of agreement on how to proceed with this. There are things he thinks I should be doing and there are things that I feel I don't need to do because it's not my responsibility. I think it would be helpful to have someone listen to both sides and workout an agreement with us. By the way, I think if DH ever stumbled across this thread he would know this about him I wonder if that would be a good thing. Maybe he would wake up from the denial *lol*