Hope someone with experience will be along soon, just wanted to say what a lovely idea!
I used to be a SW and worked with foster carers, had lots of my caseload kids in foster care. It can be a wonderful stable time for them.
I just wanted to say that I think one of the things to sort out in your head will be what you might expect your DS to be like as a result of a foster child coming in; think it through beforehand so that you are as prepared as you can be, in the interests of being able to provide a stable foster home.
For example, if you are prepared that your DS may exhibit the classic 'sibling rivalry' things, then you won't feel so bad if it happens....I think you need to be thinking that this is the same as if you had another baby. If you had another bby, ds might be jealous, badly behaved for a while, etc, but you couldn't send the baby back!
What I mean is you have to be prepared to go through with the placement regardless of probs with your DS. That could be really hard for you....but to be honest the foster child is the mosre vulnerable one than your DS because your DS is from a stable, loving home....
maybe consider short term or respite fostering to see how your DS takes it?
I think it could be a lovely thing for him though.Best of luck!