does he vomit after every type of meat? some people are more sensitive to beef or pork for example.
some other sources of protein could include tofu, tempeh, lentils, beans, seeds, nuts
tofu you can cut into cubes, roll in cornflour and fry in oil or bake. its amazing! you can flavour however you like but I like to fry then once it's almost done add pineapple juice, ginger and garlic and cook down into a sticky sauce.
tempeh you could make into like fake bacon. it's pretty tasty.
lentils and beans you could have in a tasty sauce and do with tacos or wraps to jazz it up a bit? like Mexican?
seeds and nuts can be added to anything really.
lidl and aldi are quite good for these sorts of things, so like chia seeds, milled linseed etc. add to yoghurt, cereal, salad, smoothies, etc
silken tofu can be quite useful too as you can add it to a sauce or smoothie. I sometimes whizz up silken tofu with berries and a banana for a lovely smoothie. add some of the milled seeds too if you want.
also there is protein in simple meals like beans on toast (with cheese or nutritional yeast if wanted), yoghurt (dairy or non dairy), some veg like mushrooms and peas.