What kind of things do the cousins like to do when they have other cousins around?
I agree that I'd take along books that your ds enjoys, and I'd keep them in *my* suitcase rather than letting anyone else take them away from your ds, but I'd also try to encourage my child to interact and play with the other kids. I think the suggestion to take something along such as Mindstorms or something else that kids might enjoy so that your ds has something to share is a great idea. Even taking a DVD of something he likes might work better than a book with extended family because he could invite a cousin to watch with him. Since he likes to hike, try to find someplace that you can take him hiking while you're there and invite some of the other cousins to come along.
In your op, you mentioned that your ds "is generally not interested in things kids his age like" but then go on to mention later that he's an avid hiker and likes outdoor play - those are two things that soooo so many kids like to do! Although the family dynamic might be such that the cousins tend to gather up together to play video games or something your ds doesn't enjoy, as a parent you can organize an outdoor game or hike and invite all the cousins to take part, and I'm guessing you'd find at least a few who'd join in. Or if you're traveling to an area that you don't live in, maybe there's a museum your ds would enjoy, or you could arrange a trip to the movies - anything that would be an opportunity for your ds to have fun and *also* have the cousins come along. I see two bonuses in that approach - you're helping your ds build bonds with his cousins, as well as getting around a potential conflict where older relatives think he's being too bookish.
It's one week of vacation - or one long weekend? Try to literally look at it in terms of days, and plan for your ds to have his downtime in the mornings or whatever, but also plan an activity or outing that cousins can take part in for 2-3 hours each day or every other day. My *guess* is you might find that your ds actually starts asking to spend time with some of his cousins.
Best wishes,
polarbear
