It's (thankfully!) been quite a while since I was dealing with it (and disclaimer, mine were just 'normal' fussy eaters, no autism or other issues), but what I did was: breakfasts were whatever they would reliably eat/liked, ever if this was white toast or cereal or other things that would make MNetters go 
, always with a big glass of full milk on the side. Lunches at home, similarly, whatever they liked (within reason) or when they had packed lunches it would be things they'd definitely eat even if very plain/repetetive - one of mine would only accept 'bread sandwiches' which was just 3 plain buttered slices of white bread, no other filling, this went on for years, what must the school have thought of me 
), but they were always good eaters at nursery/childminders. So by dinner time I always knew they'd had 2 good meals that day plus milk and snacks, so even if no dinner at all was eaten they wouldn't starve. Then for dinner itself, there was no cooking of separate meals, what was on offer was it with no negotiation BUT no-one had to eat anything they didn't like, and if that meant eating nothing, that was OK. Where possible I would do 'family style' serving so e.g. meat, potatos, veg were separate and that meant the fussy ones didn't even have to have the things on their plate they didn't like - yours is maybe a bit little for this yet but he's probably old enough to 'help' you serve him from bowls in the middle? I tried my absolute level best to not nag them to eat more, worry about how little they were eating esp of the veg, overly praise or reward them for eating or certainly not criticize them for not as I think this just turns mealtimes into a stress/battleground. Pudding wasn't a regular feature in any case but if something was on offer it was never conditional on them eating their first course. The only thing I was strict on was table manners, so everyone had to sit around the table for the duration of the meal even if they weren't eating, and no rude comments on the food was allowed (shouts of yuck! horrible! or throwing food around strictly prohibited as soon as they were old enough to understand). If they hadn't eaten much or at all by the end of the meal and then later claimed to be hungry, they could have leftovers reheated, or occasionally something very boring like a plain piece of toast or a glass of milk. Because I knew they'd eaten well earlier in the day and would eat well again at breakfast I never felt I was 'putting them to bed' hungry (despite some wailing to the contrary on occasion, but that would be because they wanted biscuits or something, give in to that once looking at you DH and it will go on forever!)...
It was certainly a bit annoying the times when they'd eat nothing but bread for dinner (ignoring the lovely stew or whatever I'd carefully made) and certainly there were more months/years where they ate solely beige food, very limited fruit and veg etc than I'd care to remember so this system isn't perfect, I might have managed to get more veg into them with more cajoling and pandering to preferences but I always worked full time and frankly didn't have the energy. No-one got scurvy or was ever malnourished and they're all good eaters as adults. I guess I did cheat a bit for the sake of my sanity and although family dinners were the priority we did end up doing seperate 'children's tea' and adult dinner at least a few times a week, children's tea would feature more chips and beans and peas and other such child friendly things which they'd more readily eat than more varied/healthy things which we might eat as a family, but again, balance in all things, I don't think it causes much long-term harm to eat nuggets chips and beans a few times a week if that's what he enjoys??