I agree with other posters. Since its family, I would have no qualms saying "feel free to bring a dish to share or your own Grilling item we can toss on there for you (veggie burger, veggie skewers, etc)".
I love how you will be making it all so you son can eat what he wants in his own home. I am sure that is a rare thing and that freedom for him will make for a fun day!
As far as alternate diets in general, maybe I am rude overall, but honestly I can not keep up with everyone! My circle of friends and family is rather small and the same. Even if its not a specific food allergy or vegan, etc....I have some who are the exact opposite food likes/choices and will absolutely NOT eat any veggie or fruit, unless its a potato. Seriously, hubby has several friends like this. We call them them "Carnivore Gang". But I refuse to cater to them either. I try to offer a variety when I do entertain (rare in itself) of meatless items, veggie things, pasta, meat-loaded, a few treats, etc.
I recently attended a family lakeside party. They always serve what I deem "unhealthy" foods, but offered the "feel free to bring a dish to share". Sure enough, the menu was cheap hot dogs, buns, baked beans, bulk potato salad and a few bags of chips along with a doctored up pasta salad (loaded with cheese chunks and mayo). I do not process dairy well...and my daughter and I are calorie counting, so the menu was basically out for us. So we ate before we left, and brought a few huge bowls of melons to share. They were gone in minutes! My point is, I did not expect them to alter their budget or menu for my food needs as a guest...I prepared and planned ahead (by eating before I left, and bringing something I COULD eat and share during the event)
Good luck and have a fun day!