Need Help Don't Know What to do?

Well here's the thing my name is Christina and I am going to be 22 in a week. And I finially found the love of my life. His name is Eddie. He has a son that i going to be 10 on July 23rd. I love them both to death don't get me wrong. The only thing is his son can't take no as an answer. If he asks his dad if we are going to his uncles house so his son can see his cousin who is also is best friend he cries. Yeah it had been a while since he has seen his cousin but when his dad says know we aren't going he gets upset. I mean by boyfriends uncle fell down the stairs and broke his shoulder and again my bf's son cried his eyes out cause he couldn't go and see his cousin. No matter how many times I tell him it's not the end of the world and he will see him again he always has the last word. I mean I am not his birth mother but I am his mommy. Any tips on what I can do. My son also cried his eyes out when his dad said he isn't going fishing with his daddy cause it was raining.