How about giving him the vegetable accompaniments you're cooking with your chicken, meat, fish etc and put nuts and seeds on the table for him to eat with them? Or heat up bought falafel? Obviously that won't work if you're having curry/pasta in a sauce or similar . Sigh
I'd buy milk alternatives which have added b12 and calcium.
Nut and seed butters are easy to use - there are loads apart from peanut butter; pumpkin seed, almond, cashew etc Also hummus.
I don't eat dairy or eggs (intolerant) but I eat more meat than I'd like to (fish is a problem as are pulses) so I sometimes mix a spoon of hemp protein powder (available from many supermarkets) in a tiny amount of water and neck it if I'm having (vegetable) soup. (I don't enjoy it) I also buy seaweed seasoning (very nutritious) and also packets of seaweed sheets (from waitrose) to eat as a snack. The latter are quite nice but have virtually no calories so only a contribution to his nutrition not his calorific requirements.
I think I'd be inclined to try to insist that he learn how to make lentil soup - it's so easy and freezes beautifully.
(DD (then 18) lived with a bf who decided to become veggie with the intention of going vegan. She was supportive but lamented his apathy when it came to cooking. Also he decided that anything bought from proctor and gamble or lever was unethical but rarely ever did the shopping....He got thinner and tired and lethargic and she broke up with him after a while. Apparently she knows loads of vegans her age)
I sympathise with your dilemma.