If I were your brother I would appreciate you giving me some guidance on what your 3 year old usually eats. If his kids are older he might not even remember what 3 year olds like to eat. I'd just include it in any other instructions. Shoot, I would want a list of care instructions. Not given as "don't feed my kid junk food and make sure he gets 12 hours of sleep every night" but tips and tricks on how everyone can have the best experience.
Also, as mentioned before, a week of all fried food when your son isn't used to it could create some tummy problems for him.
This is what I'd say to one of my siblings taking my kids.
Bro, thanks for taking Jr. He is so excited. Just so you know. He normally sleeps through the night and doesn't have any accidents, but if he drank anything within an hour before going to sleep, you might want to wake him up before you go to sleep to use the bathroom just to make sure there are no problems. Also, if he starts dancing while your in line somewhere, don't ask if he needs to go to the bathroom, just get him there before it's too late.
Also, his not used to eating a lot of fried foods, so when you get a chance, I'd suggest substituting apple slices for fries. That way he won't end up with any upset stomachs. If he's eaten a lot of fried foods or sweets during the day, you might want to go with something like grilled chicken and steamed veggies for dinner or even just a burger on a bun with lettuce. He really isn't a picky eater at all and I'd hate for him to get an upset tummy and ruin any of your time at the park.
And one last thing, he LOVES Goofy and will probably scream your ears off when he sees him. But he is scared of Mickey for some really odd reason so if he bursts out crying, don't worry, just get him away from him.
You are the most awesome brother and I love your kids too for wanting to take Jr along.