string cheese, fresh fruit diced up, oranges, apples, bananas, veggies w/ ranch, garden salad with lots of veggies/egg/cheese/coldmeat(even deli-slices cut up are good on salad), pickles, cheese&crackers (make his own lunchables), hard boiled eggs w/salt packets in his bag too, pasta salad with olives and lunchmeat diced in topped with italian dressing and pepper, chicken salad sandwiches, tuna salad with crackers, some guys like cold pizza. Oh and I like Mitzi's thought of cold sausage (salami maybe, a summer sausage).
I make my husband's lunch every day and will pick 3 or 4 items from the above list for one lunch sometimes. But I'd consider, if you can, to get a microwave from Goodwill or somewhere cheap and take it to work.