Suggestions above are great.
If he has access to a microwave the choice is so much greater.
Hope so, as summer is fine as can take in salad, cold meats etc. but in winter hot meals are so conforting.
If what you are cooking for dinner is suitable for a packed lunch cook an extra portion. It works out more economical than to make just a serving of something else.
It depends on what he likes really, not much use sending him off with a lovingly cooked meal which he doesn't care for.
When I was taking in a packed lunch I was lucky enough to have access to fridge, microwave and basic kitchen utensils.
I have taken in:
Casserole of many descriptions
Cottage pie
Savoury mince with a boiled egg or an omelette or white rice
Stuffed aubergine
Lentils with chorizo
Bean stew
Soup in it's many variations.
Crumbed fried fish or chicken breast with a lemon wedge and mayonnaise
Roast vegetables to go with a boiled egg or left over fish or meat from the night before
Pasties
Bread roll, buttered and filled with an omelette. Many things may be added as extras.
Bread roll, ham, cheese, salad, mayonnaise.
A small spanish tortilla, made the night before with one potato, piece of onion and two eggs.
Salads with some added protein such as leftover chicken or ham, tuna, cheese, cold cuts, or even chopped walnuts. Dressing in a small jar to shake and pour over at the last minute.
Bean Salad: beans from a jar, tuna, boiled egg, any vege of choice, olives if liked and seasoned with a mixture of salt, pepper, mustard, vinegar and oil.
A salad of sliced tomato, chopped garlic, thinly sliced purple onion, basil (optional, only if you have it) chunks of tuna or cheese.
Sprinkle with maldon salt, pepper, a spray of vinegar and drizzle olive oil.
Boiled egg, ham, omelette or leftovers to go with it.
Tin of sardines with crackers and a salad