SC, I understand that you love him and the bond that the two of you share. When making your decision as to whether or not you want to marry this man, please keep in mind that that custody arrangements change. Trust me. The most possessive of mothers can wake up one day and out of nowhere no longer wish to raise their child. I believe a man who is obsessed with his child may likely try to maneuver his way into having his child more than the 50/50 arrangement that you enjoy now. Don't expect him to admit this to you. He may not even realize it himself that he wants this. I may sound paranoid, I just wish someone had warned me beforehand to keep in mind that custody arrangements can change overnight, and you will be in a very different situation than the one you signed up for. So I guess my point is, you'll know if you truly want to marry him if you can live with these issues not going away, and his child possibly living with you every day. I think you need to decide if he is worth it. Hopefully, he is.

"There comes a time when you have to surrender the idea of what your children could be to the reality of who they are."

I have dealt/am dealing with this very same issue with my FH and his 10 year old son. He is someone who describes his parenting as "child-focused" (from what I know of it, from friends with children, he has done the "attachment parenting" thing). His son co-slept with him until last spring. When I realized that his son was in his bed on the nights that I wasn't sleeping there (he has 50% custody) I told him this wasn't acceptable to me anymore and insisted that it was either me or his then 9 year old son who slept with him. FH has described his son as his "peer," and said that son doesn't need his guidance, advice, direction, etc. When I first met FH I though how he talked about his son was sweet-- but then after I started spending time with them more and more I started to feel like the way he spoke of his son was strange, actually. He talked about the son, his "wisdom," and told stories about his growing up almost with a worshiping tone. His son grew up going with his dad to grown up parties, adult dinner parties, being involved in every adult conversation and privy to adult information. SS would demand to be a part of every conversation we had and on a few occasions told me that he I should either involve him in the conversation I was having with his dad or end it. Yet I would describe him as very emotionally immature- he still needs FH to get in his bed every night and read to him. Until last June or so he also had no responsibilities- if he was thirsty he would yell to FH that he wanted a drink. He didn't know how to pour himself a bowl of cereal. FH would take him on amazing camping trips-- last year alone, to Death Valley, to Lassen and climbed to the top, and in the wilderness of Shasta. Yet his son never helps prepare for the trips, doesn't take anything to/from the car. Doesn't help putting up tents, preparing food, cleaning up the campsite. I know this only b/c I started asking questions after the camping gear sat in the living room for 6 weeks last summer. SS doesn't ever put his dishes or glasses in the dishwasher, he would never offer you help. Yet I am supposed to be amazed by his writing abilities (FH is a poet and writing professor, BM is a published author), amazing by everything about him.
Well, I have spent 6 months trying to point out to FH the things that I think SS needs to learn, why he needs responsibility and structure, and why he needs to be a kid (he has never had a friend over in the time I've been with FH; part of this is where we live, but his time with FH has always been just about 1:1 adult time). I have stressed to FH that I am not going to compete with SS to be his partner. There are many times I have erupted in anger, many times I have felt isolated, and many times that FH forgets about me. But finally, after 6 or so months and starting therapy together 2 months ago, FH is starting to understand. We have had some of the first healthy and productive conversations in this area recently. He used to get so angry and resentful- and would characterize it as just me requiring him to change the nature of his relationship with his son, that I was asking him to make changes that were damaging, etc. We are finally in a space where he sees that our relationship is improving, and he wants it to continue to.
I sometimes feel bad- that it is a big change for a kid (to have his relationship with his dad shift so much) and that SS didn't ask for his parents to split up when he was a baby. But truthfully, I believe it is much better for him to be a kid, for him to be allowed and encouraged to do things that kids do, NOT to be expected to act like an adult, but encouraged to become responsible, take care of himself and his things, etc. And the truth is, I need to have a fullfilling partnership. I left one marriage b/c it could not sustain me in the way that I needed. I most certainly am not going to enter into another that can't either.
So yes, I suggest therapy together. Someone recently rec'd the book _Step Coupling_ to me and it was helpful. Your FH needs to see himself and you and the primary dyad, which will for sure be a difficult transition for him to make.