Well, a couple of things. I'm personally a little old school and think kids do have every right to want ALL of their parents attention and dating and moving in with a new woman and her child would make for some hard feelings. It's just the way it is and why dating someone with kids is complicated and relationships often fail because of it. But sometimes it does work. Understanding and not expecting too much from his son and encouraging your boyfriend to put his son FIRST over and above you and your child will help Tell him to go out and do things with his son without you guys. Let him be the disciplinarian. Let him have time just with the boy where you and your son get busy doing something else. Make a huge deal about how he is your boyfriends child and how important that is. Don't judge or call him antisocial or wish he were different than he is. In fact, he's probably much different at the house he spends more time at!
I have a 14 year old son. They are NOT the most charming creatures. Awkward is one word to describe it. I also have a soon to be 13 year old. He's still little kid like. With that one year age difference, everything changes. It's a tough age. My son is sulky and moody and hyper sensitive, and self conscious. He's hormonal. And sometimes a pain in the butt. LOL But it is ALL normal and right on target for the age. Not a great time to meet him if you are going to judge his behavior. My boys is so sweet, I can't tell you. I love him to pieces. But he's not always the sweetest right now. And that's what it's like with a young teen boy.
And reality is that he probably does prefer to be with just his dad. That's NORMAL. You have to understand that. Something else that is REALLY important is that these are very important years for this teenage boy. He's developing his sense of place in the world. He needs a strong relationship with dad! He needs to feel good about himself. And anything else puts him in a position to be at risk for things that are scary to a parent. These are the years in which kids can get a direction in their life. Is he going to go ahead and drink the beer offered him? Is he going to try at school or skip classes? Is he going to smoke? He needs as much parental support right now as his dad and mom can give him or he is absolutely at risk for trying drugs and other things that can be a tragedy for teens.
I'm not trying to discourage you or sound harsh. LOL I promise. I hope your relationship works out. but this is an important time to see the bigger picture her of this boy and what HE needs. And your boyfriend will love you all the more if you focus on that. good luck!!