Oh boy, sounds like you need advice or something just to keep you sane.
I think a lot of this is jealousy on both boys parts. I think your son feels that this other boy is moving in on "his" territory, taking "his" parents, moving into "his" home, and now has to share "his" bedroom too. How long has he been with you? Your foster son I mean? Has it always been this way or is it just cropping up now?
I dont' know what to tell you other than to sit both of them down and explain that like it or not, they have to learn how to share and get along because "he" (the foster son) isn't going anywhere. He is staying. Then you have to tell the foster son, that he is to respect your son's things etc. because it was his home first. He is willing to let him live there and share his bedroom etc. so he should be happy about that.
Tell them they just have to learn to get along because they have a lot of years yet to live with each other. How would they feel if you and your hubby went around doing the same thing those two boys are doing? Think it would have any reaction on them? Worth thinking about.
I wish you luck, maybe talking to a counsellor can offer some

to you as well. I never had that predicament to be in so have no

.
DeeDee