Caring for a child with a hearing impairment...

[Bookworm]

I think you are being too hard on yourself! You are doing everything you possibly can to help this child develop and learn and that is commendable. You are willing to go above and beyond to learn new things to help this child out, and what you are learning and doing is working. Take satisfaction in that.
I think if the family is happy to keep him there, and you are happy to keep him (hearing aside) then do so. He is going to have to learn how to live in the "hearing" world, so he may as well be where he is comfortable.
However, if he is severly struggling and you can't cope, then I would say terminate. However, I think from the sounds of things it is going fine. It is obvious you are a very caring provider and that little boy is lucky to have you.