@toiletpaper. My Dniece is now in her early 20s and is an extremely fussy eater, DS's friend is autistic and had very strong preferences for food (think ham sandwich and curry every day for years), DS eats most stuff but doesn't like lots of milky/creamy textures and won't touch chocolate. But he was very much into the 'kids food' up until a year or so ago and he'll still prefer pizza or nuggets if we have a curry (I'm not worried about that now, he eats everything we do apart from that).
What we've all found very successful is doing a meal they like but putting a little taster plate by the side of what we're eating. My friend managed to get her son eating roasts, cheese sandwiches, wraps, fish and chips, bolognese, pizza (which has increased his diet quite a lot from ham sandwiches etc) like that. My Dsis did the same with my niece and she's now much less fussy. I increased DS's food choices doing the same.
If we go on holiday DH and I always put a little of what we're eating on his side plate. We got him onto paella and mussels when we were abroad last time by doing that.
It's literally just a spoonful or two of whatever you're eating, on a side plate. Don't force it. If he doesn't want to try that's fine. If he does just point out that you're really pleased he tried it, did he like it? Then leave it. If you're doing a meal like that again put a bit more on the plate. If he eats a spoonful, that's fine. If he doesn't that's fine too. It's getting tasters going.
DS this week has had beef casserole, sausage toad, fish and chips, chicken casserole and burgers.
Things that go down really well are spag bol, sausages/sausage toad, sausage meatballs in the same sauce as the spag bol (just get sausages, skin them, break into thirds, roll to a ball, fry, stick them in bolognese sauce) - I serve them with rice but I've also popped them into a pasta bake. I don't do sausages/toad/meatballs in the same week just to spread the processed food! Fish and chips and burgers (again I make these with sausages and paxo mixed together, made into burgers then fried or oven baked). And any casserole with dumplings works well!