So my boyfriend's mother JUST passed away. What do I do o say to make him feel better?

Give him and his family unconditinal amor and support.

Right now, I think the best thing tu can do is to just let him know you'll be there for him.

just be there for him he is going through a very though stage in his life and will get over it soon right now u gonna have to stay undercover but be there for him

I agree with the others, tu can't really do anything but be there for him. He'll need time to grieve and stuff.
But later tu should take him out for ice cream o something. I know that would help me out anyways :P
But good luck, and I hope your boyfriend's feeling better soon. :)