First, I think it is so sweet of you to do this! I'm sure his mother will greatly appreciate it!
I'm a type I diabetic & have been for 30+ years. Anything "combined" (like casseroles) are difficult, but not impossible. It's best, though, to stay away from them for now - while they figure out what works & what doesn't for him.
"Plain" food is best - roasted meat (beef, pork, chicken) with potatoes & veggies on the side. Baked ham is another good one as long as you don't rub brown sugar on the outside.
Assuming he's on insulin, starchy foods will be limited but cannot be ignored altogether. However, there are some veggies that count as a starch/bread item: corn, peas, lima beans. It would be better to take some sort of potato dish and green beans or carrots. How about scalloped or mashed potatoes? Just don't add cream cheese to mashed or a bunch of cheese to the scalloped - too much fat is also bad. I also stay away from boxed scalloped potatoes - did you ever read the label?!?
Meatloaf would be okay as long as you limit how much ketchup you put in it. Ketchup has too much sugar in it to be considered a "free" food the way most of us think about it.
If you take a garden salad (we eat lots of those because most raw veggies don't really count) be careful about the dressing. Most low-fat dressings have increased sugar content - evidently they use it to help with the consistency when they reduce the fat.
You might want to consider taking along a Jello salad using the sugar free stuff. I don't think it tastes weird with the new sweeteners (not like 30 yrs ago - yuck!) and it will seem like "real" food. Or even Knox Blocks using the sugar free - as they're always a hit with kids.
Whatever you take, I'd suggest that you provide the mother with a list of everything that went into what you bring. It wouldn't have to be the full recipe, just ingredients. Something like:
ham, cloves, mustard
potatoes, flour, milk, margarine/butter substitute/whatever, salt, pepper
sugar free jello, pineapple packed in juice......
Then she won't be left to wonder if something is truly "safe" for him or not.
I mention that because when I was first diagnosed I went to eat dinner with my grandparents. My grandfather was also diabetic. So when the Jello salad was served it never occurred to me that it wasn't okay to eat it...especially when I saw the pile Gram gave Grandpop! Well, I ended up in the hospital because she'd used
real Jello. Here she'd been giving it to Grandpop for years!