hmm, will his children finish their education in 2 years?? What about college, university? Perhaps his wife will get ill and of course he can't leave her then can he (sorry cynical moment). What are you going to do then? Waste you life for x more years because you really love each other for a man who may never ever be there for you.
Tell him to either make the break now or to go away and come back in 2 years when his kids have left school and he has left his wife.
No matter how dead HE thinks their relationship is you have no idea what his wife thinks. She could think it is all happy and rosy and they could be having sex twice a day for all you know (despite what he says to you). She could be looking forwards to in two years time having her and her husbands lives back so they can go and do what they want childfree.
You have only to read many of the posts on here to know how hurt, betrayed and deceived some of the women have been. How much this has affected them and their children.
You, your child, his wife and her children all deserve much more that to be treated like this. How do you think his children especially are going to feel when (if) they find out they have a brother?
I think you were stupid to start a relationship with a married man but hey we are all stupid sometimes (especially me). It's what you do NOW and for the future that can rectify that situation.
Also bear in mind that his wife is probably going to hate your guts and do everything she can to hurt you. He children will probably feel the same for a long time.
Wow what a waffle, good luck for whatever you do.