The way that I handle things as a wife and BM is that if there is an issue- I talk to DH first, make sure that WE are on the same page. Then I address it with my kids father. If we all aren't on the same page I tell him to talk to his wife, I talk to my husband and we come back again a little later. As the custodial parent, I have to make a decision that is respectful of my kids father's wishes but make sure that it isn't disrespecting my husband either. I have to choose what works best in my home. (Thankfully, we haven't had many issues that we don't agree on). 

So if you're going to break it off, I'm not sure this has to be resolved. But if you're considering staying, then I think you really do need to sit down and talk out what has been bothering you and what you need to be involved in. I'm not gonna lie I straight up made an Excel chart about household things to talk through and what each person was responsible for. I often give my advice but then back way off because it's not my kid, but I am very very very separate and disengaged. I get no one ready for school. I do no parent teacher conferences. I am a bonus adult in their lives, but not a parent. I expect the parents to parent.

Stop doing the hard stuff. The kid has 2 parents to do for him AND to financially provide for him. Stop letting yourself be used.

Save yourself the future headache and break it off. There are plenty of stories here that will give you a possible glimpse into your future and it isn't pretty. Find someone who does not have a child and enjoy your life. 

So she wants you to do her job as a parent from all angles but doesn't even have the grace to keep you in the loop about her son? Why is she in such a hurry to run to the guy who cheated on her multiple times with his other women? Especially when she's been angling for a "family" with you?

Stop doing it all. If she wants someone to get her son ready for school because she can't, let her hire a nanny/babysitter. If she wants someone to help her son with his homework because she can't (tell me, why are her days so long that she can't be present either in the morning or in the evening for her child?), then it's time she started looking for a tutor. Don't do any more for her.

She'll only notice how much you do for her when you stop doing it.

And tell GF that not only are you not providing for her son financially, you won't be parenting him either.  So she needs to get him ready for school, arrange for drop-off/pick-up, etc.

Honestly if the bio-dad is in the picture so little, she will be a single parent.  Are you sure this is the relationship for you?  She will be off to a parent-teacher conference (maybe with ex) while you are doing...what?  At a happy hour with work friends?  I'm not sure your lives are in a place that is very compatible. 

I think you need to look for a relationship with someone without kids (or at least - much older kids that don't need daily parenting anymore).