Oh, I really sympathise with your mum, I have had to cook for people like this too, not easy. If he lives with them then I would suggest that he cooks his own meals, its not like he has special dietary needs or is a veggie or vegan.
Personally if he was just there for one meal then I would make something he liked but if he is living there then 'tough t*tty mate, sort your own food out, its not a hotel'. Why should your mum be stressed out because a grown man does not like vegetables, he is happy enough to shove junk food and e-numbers in his gob, so for him I have no sympathy whatsoever.
Sorry if that sounds harsh but as had been said, he should be grateful someone is making him a home cooked meal.