Should my very soon to be husband enforce his son to include me in conversations?

I have tried everything to get along with this boy. He's 11, it's been a year that I have been living with his dad. His mother tells him bad things about me. It's a 50/50 custody. So during the weeks that we have him here, it's almost unbearable for me. 95% of the time he won't direct his questions or conversations at me. He doesn't ask me for help. He basically doesn't include me in anything.
I have tried so hard. I have tried to have conversations with him about his friends or school. I've taken him shopping, out for walks. We've gone to the big exhibition in the city. I've made him his favorite dishes for supper. I recently stumbled across texts that he sends to his mom and the things that were said back and forth were outright mean. So I took away the iPad that I had given him.
The biggest issue that nags at me is his dad doesn't really seem to care. We've had lengthy conversations but he blames it all on his ex and says "I can't do anything about her". Should he maybe force his son to include me and if he doesn't then give him consequences?
I'm seriously having doubts about getting married.

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