Babysitting a kid with behavior problems

I babysit my nephew who is 5 years old a couple times a week because my brother and his wife have work schedules that overlap. This kid is one of the worst behaved kids I've ever met, especially when his parents around. He will kick my dog in the face, refuse to eat if he doesn't get what he wants and says the nastiest things. He even got kicked out of daycare because he was hitting little kids. He also hasnt learned anything teachers try to teach him. His teacher told his parents that he needed extra help because he refused to write when they practice ABC's so I try to help him when I babysit but he doesn't want too. He gets frustrated if he plays any sort of game and starts hitting himself and he only draws really disturbing pictures (he drew a picture of someone killing someone once) Doctors already told him mom something is wrong with him and he might have anger issues but his mom hasnt done anything about it. I don't really want to babysit him anymore but I don't want to cause problems.  How can help this kid??