Can, remember if you don't ask for what you want, you don't give people a chance to give it to you. I do think you should let him know you want to buy the house and if it comes up that you have a preapproval. In your situation, he would probably think you couldn't qualify to buy him out and you waste a go-around on that. Of course don't disclose the amount. You are of modest means and have a child to support. Even if you get the new job, you have little recent job history. This guy should not be thinking he hit the jackpot. He should be thinking how much do I need to make sure my kid has a nice home.
"00 you raised the issue of selling the house and after thinking through the options and the cost of housing, I'd like to buy you out to make sure D has stability and a nice home. Please let me know what you think would be fair."
Again, you don't have to agree with him. But if he lowballs himself, as some have, then that is best for D and it could be a quick discussion. If he doesn't you have time for all the appraisal talk, negotiations, etc.
I know he hasn't acted like your H for a long time, but somewhere in there is a person who loved you, married you, had a child with you, and likely wants you to do well and be safe. In my worst days with OD, I've always believed that he does want me to be safe, to have a good home, to be able to offer refuge to my children when they need it. I still have faith that in the end, he will be kind even if the road to get there is twisted and rutty and tough. Hope is the thing with feathers. Don't borrow trouble and assume the worst.
