Beanburgers - not the crappy meat imitation things, as they are horrible in taste and texture imo, but proper beanburgers with texture, a crispy coating and all the usual burger additions (cheese, tomato, lettuce, sliced pickles, etc, etc).
Chilli - even the Stagg tins are nice and you can easily increase the volume and nutrition by adding further veg to them - they're very good for jacket potatoes as well.
He might not like dahl/lentils, but he might like chickpeas (the tinned brown ones have far more flavour and texture), kidney beans, black beans or butter beans, for example. If he likes tomato, they lend themselves far better to things with a large amount of tomato puree added, like chilli with black beans/kidney beans, etc. Adding things like tamarind paste add extra depth of flavour than can be lacking in lentil dishes.
Myriad spiced vegetable dishes - cauliflower, okra, potato, aubergine, courgette, mushrooms, etc.
A jacket potato with cheese and beans (with or without veggie frankfurter sausages) is a good meal - you could also do sweet potatoes and cream cheese instead. At risk of a cliché, lots of salad goes well.
Nothing wrong with things like egg & chips, vegetarian toad in the hole or halloumi burgers. A cheese and onion pasty or a vegetable pie is fine (the latter is good with potatoes and standard veg. And if he likes quorn, there's all the nuggety things.