I'm pretty big on making sure that I, DH and DD have nutritionally complete and healthy meals pretty much always. But, I'm also of the mindset that during a vacation, having to think about calories and fat all the time sort of ruins the experience. (I actually like grilled chicken/fish and fresh veggies anyway, but don't stress if DH and DD spend a few days eating junk if we aren't at home).
Personally, for me, if I were sending DD (also 3) away for a week, I'd be more concerned about her being happy and comfortable. If her cousins would all be eating french fries, I know she'd notice if she was told she had to have broccoli...and be upset about it (potentially making it more difficult for the adults caring for her). And yet, I do get that when you allow your children to be "spoiled" by relatives, then undoing all of that and getting them back into their normal routines is tough...so it's a balance. And a week is kind of an awkward time...it's definitely a long time to be eating nothing but junk food and definitely long enough to have them get used to an alternate diet, but not quite so long that if they did eat junk the entire time, it would cause health problems (other than maybe an upset stomach from sugar and grease).
That being said, I think it's fine to talk to your brother about your son's diet. Just say that you are teaching DS about healthy food choices and want that reinforced on the trip...if they are at a restaurant where he can have veggies over fried foods, then that's what you'd like ordered...and this especially if he won't complain that his cousins are eating something else and asking for that.
As for your DH, I hope he understands that this is an opportunity for him to bond with his uncle and cousins and his uncle is going to take good care of him. If you want to request a certain diet, I think that's fine, but I don't think that the diet/food choices alone should dictate if he goes or not. I also like the idea of offering some money to cover the meals that you do want him to eat.