I sometimes take care of my nephew (
and niece (12) for up to a week at a time. My SIL, their mom, is very careful about what they eat, cooking everything from scratch and really limiting candy and sweets. When I first started taking care of the kids, I was asking her about nearly every meal.
Now that I have a better feel for what she wants and now that she knows I won't be stuffing the kids with candy to keep them quiet, these are the rules that we've come up with:
There will be one meal of hot dogs, mac & cheese from a box, and peas. This is their all-time favorite meal.
The kids will be taken out for ice cream one or two times.
The kids will be given at least one fruit and one vegetable daily. (Although I don't promise they will eat it. I am not forcing the kids to eat anything. Fortunately, they eat pretty much anything I put in front of them.)
We will bake one batch of cookies and the kids will get to eat some. (The rest go home with them as a present for Mom and Dad.)
The kids will get one piece of candy every day, after lunch, from Great-Grandma's candy bowl. This is a family tradition, starting with my grandmother.
Because my niece gets very cranky if she doesn't eat at regular intervals, SIL provides a big bag of healthy snacks for both kids.
So, I'd come up with some minimal requirements for your son's meals, like one fruit and one vegetable per day, and limited sweets. If there's anything that would make your brother's life easier, like "He needs to eat every three hours or he throws massive temper tantrums," or "If he eats too much greasy food, he'll throw up," please tell your brother that, as well. Send him off with a big bag of healthy snacks.
If that's not enough for your DH's peace of mind, then maybe he shouldn't go.