My opinion (only that) - you Can't make it clear to the mom. It is her child. She has to make the decisions and live with them.
I know you love this kid and really want what's best for him, but other than telling the mom straight out your feelings, there isn't much you can do to change her mind.
She must think he's in the best environment right at your place!
That's a wonderful compliment! I know you care, but sometimes we have to let the parent be the parent.
Just IMO.