You have my sympathy. At one point I was feeding a veggie (me), a picky teenager, DS (GFCF), a carnivore and a mum who didn't want 'any of that foreign muck'! Admittedly our budget was a little higher.
I would suggest that for your ds you start with just GF, because if you do see an improvement you won't know what's causing it. You can get basics (gf pasta, bread, flour, etc) from your GP.
We have a lot of veggie curries, pasta dishes (boil 2 types of pasta), shepherds pie, soups, home-made pizza (2 types of base), etc. If the boys want meat, eg bolognese we just do a basic sauce, tip half of it into another pan and add the meat. Omelettes are good too.
You can get gf fishfingers/sausages, etc but they're really expensive. Try your own goujons? (GF breadcrumbs are not bad!).
Puddings are harder - we eat a lot of fruit. Generally everyone's diet improved once ds went gf. Treats like biscuits and cakes I'm rubbish at so I do buy a few gf items - but it is possible to make them.
Watch out for cross-contamination - use seperate utensils, don't put gf bread in the toaster, etc. Thankfully we all muck in with the cooking and washing up!
We did notice an improvement in ds's behaviour - less aggression, more able to concentrate, by it was by no means a miracle cure, so parents trying to make you feel guilty can fuck off.
We started off gf/cf but after a few years gradually introduced cf. I think this has increased ds' prone-ness (?) to constipation but that can be dealt with fairly easily (hoorah for chocolate flavoured movicol) and his diet is so much more varied now.
It sounds a bit daunting but it's become such a habit now, I can't imagine doing things any other way. Good luck!