My husband doesn't think I should consider his kids my kids too.?

Example, my husband's 16 year old son is going to be visiting us for a week soon from another state. There is a tae kwon do tournament that will be happening in our state that my stepson will be participating in. I was excited to go watch this tournament. My husband, however, wants alone time with his son, and doesn't want me and our daughter, and my son, to come and watch. He is a stepfather to my son...yet thinks of him as his own. I think of his son, and his daughter who is 5, as my own kids too...yet he doesn't think I should. I understand that he wants alone time with his son, but I really feel like I am being excluded from this situation. Telling me I can't go watch my stepson perform is like telling me I can't go watch my own biological son. I feel like my husband needs to realize that he has a wife and other kids to think about...and not just himself. The tournament should be a family event in my eyes. I don't understand why we can't all go, then him and his son can have more alone time doing other things. Help!