Yes it absolutely is possible (we have been a mixed meat eater/pescetarian/veggie) household for a while now. You do have to be tolerant of everyone occasionally eating different meals and this will mean either the cook putting in slightly more effort or someone (usually the DC!) eating more ready cooked/beige food than is ideal. Judicious use of the freezer and careful meal planning is also essential!
However to encourage you, I expect your DSS will get less fussy with time as well and perhaps you may even persuade him and your DH to try some veggie food occasionally too (my DH is a massive carnivore but I'm gradually bringing him around to the idea of a few veggie meals a week!). Leaving aside the environmental and ethical issues, veggie can be a really fresh and healthy way to eat and saves loads of £££ so I'd really encourage you to give it a go.
So for the meals you mention, here's what I'd do.
Chicken fajitas: everyone would have the same wraps and sides, veggies have some spicy beans or possibly quorn/tofu strips in place of the chicken (although we don't really like these so avoid them where possible!). I cook the beans/quorn in the same spice mix and with the same onions/peppers as the chicken but in a separate pan. This does mean two separate pans of filling but cooked at the same time so not much more effort.
Chilli or spag bol: meat eater have normal versions, veggies have three-bean chilli and lentil bolognese. To avoid lots of additional prep I tend to batch cook these, so e.g. if I've made a big pan of meaty chilli from fresh, the veggies will have 3 bean out of the freezer with the same sides, and an extra portion of the meat version will go into the freezer for next time, or vice versa. We do this with loads of family meals, curry, shepherds pie, lasagne, moussaka, tagine, stew, burgers to name just a few, all can easily be made veggie and frozen and then served with the same sides as the meat versions - happy to share recipes for both meaty and veggie versions if any of this floats your boat?
Pasta meatballs - the veggies would probably just have the pasta with a tomato-y sauce and lots of cheese TBH. Not the most nutritious as lacking in protein but tasty!
Chicken/Tuna steak and vegetables - I would probably do a pepper or a butternut squash topped with veggies, grains, nuts and cheese to replace the chicken/tuna here. Then serve with the same sides. Or perhaps for more of a treat/easy meal the veggies would have a slice of ready bought or pre-made and frozen veggie tart, quiche or pie. Or perhaps a nut roast or something quorn-y like quorn sauages or 'chicken strips' if you like them?
Hope this helps!