PLEASE HELP! My fiance's 8 yr old son is a spoiled,disrespectful brat. How can I make this work?

I'm getting married this summer to a wonderful man. The problem is his son who he has joint custody of and lives with us 50% of the time."Tom"is a only child and VERY spolied by both his parents(his own grandparents think this)there is no discipline or time out for bad behavor.Their way of dealing with it is to innore it. I'm sorry but i have a hard time "innoring it' when Tom is hitting me with a pillow when i'm on the phone! If I try and tell my fiance something Tom is yelling and climbing all over his dad trying to get his attention he hates it when hes not the center of attention 100% of the time. Tom also like doing only what he wants to do and will talk back.If i ask my BF to help me get grocerys Tom will jump in and say rudely "You go get them yourself dad and I can stay here".I know its not Tom's fault its the parents but i'm not 1 of the parents so how should i hanndle this?I have tip toed around the subject with my BF but in his eyes his son is perfect and he sees no problems