Have you talked to him about it? I would tell him that you feel you go out of your way to include his kids sports activites in your life and he should do the same, which he should, and not its not fair. Especially if they are not conflicting and its do-able.

You can't force this. It has to happen naturally. I go to all my son's games and DH used to go to his kids, when they were in sports. Now it is Karate. You have to take it slow on this stuff. Don't go to every game with him. That is setting a precedent that you will not want to keep up, when things get difficult with blending.

So you want to convince your BF to attend your kid's activities and NOT his kids'?? I think each parent should attend their OWN child's activities if there's any conflict.
You said you go to see his kids when yours doesn't have sports...do you go when he is with his father? Is his father around? Do you miss any of your son's games in order to go to the skids' games?

I would attend my kids' activities...regardless of "whose" time it was.

ETA: Saw your comment about your son's father. He's not around, which means YOU go to ALL of your sons' games. Yet, you want to your BF to miss his kids' games.
C'mon, ferreal??

There's no such thing as "blending" As the book Stepmonster puts it: "when there is blending going on someone gets creamed" or in so many words.

There is a fine line here. Number one being nobody likes to have to share their children after divorce. And certainly not miss out on things their children are doing. I have done this a few times. It kills me to miss my son's games etc. I think he wants to watch his kids in sports just as much as you want to watch yours. And he wants to watch your kids about as much as you want to watch his. Lets be honest. It's not the same feeling. I too have felt like an outsider when it comes to my skids. And I too felt hurt a few months ago regarding us spending time with my son during a weekend we both decided we'd go watch his games. Then something came up with his own kids. How can I tell him not to go be with his kids when I so badly want to watch mine? In order to blend it doesn't mean it's "our" time. Kids don't understand that. Hey I'm not coming to your games because it's your mom's weekend? Can you imagine the feeling a child would get? I look at it like this just because you are divorced and have to share the child doesn't mean you have to split everything up and not partake in your child's activities simply because it's not your weekend. If you were a full time parent there would be no option regarding this.
I got great advice here regarding this type of situation. Take this time with your child and enjoy it together. Do something special after his game. It's about the kids. Blended families will happen but not over sports or other activities. It will happen over dinner or other special times together. I have a blog I posted when I was feeling some what like you are. And after I wrote it I got great advice on here. We all love our children the same. Just because he chooses to not miss time with his boys doesn't mean he doesn't care about your child. It just means he is a father who doesn't want to loose touch with his kids. That is a good quality. Just as you wanting to be there for your son is a good quality.
Stop going to all the skids things. Make time for yourself or a friend to do something fun together. Fill that time with a few good laughs with some friends. OR go. But either way understand that blended families aren't perfect and they are work and be proud of a man who doesn't just want to see his kids on "his" scheduled time. And be thankful right now those skids don't hate your guts.

I wouldn't even mention his going to his kids games on their mom's time, because that might make him a lot less willing to listen to you and compromise. What I do think you should point out is that you make an effort to go to his kids' events, and it would mean the world to you if he did the same for your son.

At 11 and 14, these kids are old enough to understand this, but I can guarantee you that they likely do feel some jealousy over the fact that your son gets to live with their Dad full-time, regardless of the fact that his dad is not in his life. Like I said, they're old enough to understand this, but preteens/teens can be really hormonal, moody and selfish. I think a lot of this depends on how BF handles the situation.

I think that everyone in this situation is going to have hurt feelings at some point that need to be addressed. For your son, that might be that he feels jealous that the stepkids have a loving, involved dad, while he doesn't. For the stepkids, that might be that there is a new kid that lives with Dad full-time and they now have to share their dad with him. That might mean that he winds up not going to as many of their activities.

I think BF is going to have to address the feelings his kids have by talking with them. Life's not always fair, and this is something he needs to explain to them. He will always be their Dad. He will always love them. He is not trying to replace them with your son, and you and your son aren't trying to eliminate them from his life. Your son is now a member of the family, and deserves to be treated with attention and respect. Everyone in this situation is making sacrifices, and everyone has to make do with situations they don't like. They can always come to him for advice on this (and any other) situation, but they cannot be mean and rude to you or your son, and must treat you both with respect. I think you should talk with your son about this situation too. Does he ever feel jealous towards the stepkids because their dad is involved in their lives? Maybe this is something you could address with your son. It's ok to feel jealousy, but he cannot take it out on or be mean to the stepkids.

My BS17 still expects me to be at his games. I guarantee his kids expect the same from him...It is hard to disappoint them. If he has nothing else it is fine to expect him to be at your sons game. If his kids have something, he should be at theirs.