I feel your pain - I am vegan but (or maybe because of this!!) DD (12) has gradually cut more and more plant based protein out of her diet and now, apart from mung bean soup and lentil soup (which she’ll accept sporadically) will only eat animal protein.
she gets bacon once a month as a treat. We’ve recently gone over her likes and dislikes and now her diet looks like this which is a compromise because I don’t think it’s good for her, the planet or my bank balance for her to have meat every meal:
Breakfasts are always vegetarian and things like a banana and almond butter smoothie, boiled egg and soldiers, porridge (which I add flax seeds to and make with oat milk), quesadillas
Lunch she packs for school and often sadly lacking in protein as she’ll only take in tinned tuna (and again I don’t think she should be having that more than twice a week) or a yoghurt so that’s when she loads up on fruit and veg and usually a carb like brown rice. She’ll buy a cheese and tomato panini from school on occasion.
Dinner is very much a meat and two veg affair (she thankfully likes veg and usually raw so at least that’s easy to cook!), she has steak maybe once a fortnight, otherwise it’s chicken drumsticks, salmon, white fish, mussels, clams, sometimes she’ll entertain lamb or pork. She will also eat lasagne and spag bol (which I bulk out with lentils and make the bechamel out of cashews for more protein)
If we go out for Sunday roast she ALWAYS chooses the beef!
hope that helps a bit! I would def see if he’s willing to substitute some meat for fish - still terrible for the planet but at least better for him.