Sorry that you're in a rut, though it's nice to see the creativity at play here with the standard school lunch. Here are some other ideas though it may require a more adventurous palate that your kindergartner may possess:
Siu bao - Chinese steamed buns. They can be eaten room temp or hot and you can just get some of the takeout soy packets. I believe you can buy them in the frozen food section at most Asian markets.
Cold fried chicken. Mmmm.
Taco salad i.e. taco meat, chopped romaine lettuce, shredded cheddar chips and jar salsa - just pack the ingredients separate and have him mix at lunch.
An alternative to sushi would be kimbop (sp?), a Korean nori roll consisting of nori wrapped around rice, bulgogi, some vegetables and pickled radish. Very tasty and not so susceptible to spoilage. You can get them at Korean stores like Chicago Food Corp. or Clark Market in the hot food section.
Assorted crackers cheese and fruit with maybe some sort of meat like salami or ham.
In the warmer months you could also turn the hot thermos on its head and pack a cold soup like gazpacho.
Sandwiches - Italian subs are a nice departure from the standard wonderbread sandwiches. Also, if you have steak or roast for dinner these leftovers make for really great sandwiches too. Or you can take the pita concept on its natural permutation of pita pocket sandwich.