I'd say both: don't get all worked up about it, he is going through his own thirteen year old stuff right now, which you might not know anything about and he is being private. Or not that, he is just distracted with lots of other stuff, unless you are right there. He's finding out who he is in his mind, as happens with thirteen year olds.
Chances are good you can be some kind of friends for life, but don't push it, just be there for him if the occasion arises.
I'm a way older woman, and have had two close cousins, females. In our teens and up through our thirties, one was closer than the other, but that evened out over the years. The two of us who were most different in interests were just plain glad to be related. Then, way later, that one that had been closest shut both her sister and myself out, only wanting to deal with her own immediate family. It was all quite strange since sister and I couldn't figure out what triggered it all, we'd always been who we are back when she liked/loved us, but the one who wanted out of family get togethers or conversations by some kind of mail had become a psychology expert in attention deficit situations, and self regulated herself to not taking on more than she could deal with. She grew to be superorganized, and I figure her sister and I were the least of her changes.
So, these things aren't always about you, the one not getting full attention.
As it happens, I still love both cousins and all their children.
Also, seventeen and thirteen in the teens is quite a span of years, and that same span means just about nothing different as ages progress.
Don't be worrying. You have a cousin you actually like.
Edit - I didn't have any brothers or sisters and my family moved a lot, so I get you on that.