Grandmother is turning my son into a monster!!?

I know this situation all to well. It's hard to say something to the grandmother I know but maybe you could just ask her to cut back on the prizes. You could say something like I'm not able to buy all the stuff you do for him and he thinks I'm being unfair to him so if you could just limit the gifts. Also persistance is a big thing. Don't give into him and it make take a spell but he'll eventually realize you're not going to give in when he throws a fit. Hope this helped good luck.