I'm sorry he's going through this. I think that is so nice of you to offer to watch your brother for your mom.
I still think getting him a fleece blanket (big one) is a good idea and something he can use.
I'd still get him the snacks even though he may not feel like eating them right away.
Get him some books of his favorite authors (he may not be up to reading them right away.
A couple of certificates for restaurants for your mom to eat where the treatment center is.
Some letters written to your stepdad that he can read later when he's going through treatment that will lift his spirits.
One small book of those positive thoughts or affirmations per day type of things.
Just your calls, offer to help, your prayers & your company would be best as well!
I hope he gets through this trying time!