Why doesn't he eat much during the day? Breakfast and lunch would be good places to add in some calories. And if he was eating regularly throughout the day, he might not be so exhausted when he gets home.
Breakfast could be a muffin. You can get a 6 pack of high-fat muffins at just about any supermarket bakery. Instant breakfast on the go. There's even pre-cooked bacon available now.
Breakfast doesn't have to be breakfast food. He could have a sandwich or a salad or a bowl of soup.
Lunch could be a simple sandwich, a sub he picks up from a shop, or even a Lunchable. They sell packs of tuna with crackers for an grown-up Lunchable. Or those instant noodles in a cup, if he has access to hot water. There are tons of things that are pre-packed in single servings. Mostly they are marketed towards kids' school lunches, but I see a lot of adults at work eating them--single serving puddings or Jello, applesauce, canned fruit.
I think if he can make the behavior change to eating more during the day, it would be a big help. Even just a can of Ensure for breakfast and lunch, or Carnation instant breakfast.
As for freezer meals, I find tomato-based dishes freeze well. Chili, spaghetti sauce (with or without meatballs), casseroles with tomatoes and rice rather than cream and pasta. True, he'd have to cook the pasta for spaghetti, but it's about the same amount of work and only a little bit longer than nuking a frozen dinner. Do a pot roast with veggies in a slow cooker, or roast a chicken with some veggies and then divide up into single servings of meat/veggies and freeze.
I'd work on getting enough calories into him to start. Once he's accustomed to eating enough food, then work on the nutritional aspects of what he's eating.
Is the lack of eating enough a new thing, caused by some event in his life, or has it been going on for some time?