Most important thing is to shut your ears to 'advice' from anyone who isn;t either the parent of an autistic child or a specialist. Because they haven't a clue. I got so frazzled, being told he was being naughty, I was spoiling him, if he;s hungry he'll eat etc. All rubbish.
Give him lots of bottles of milk, if that's all he'll take (DS had bottles until age 5.) See if you can get one that is higher calorie, or mix some single cream into it (half a teaspoon the first time, introduce it in teensy amounts so he doesn't notice and increase the amount every three days or so.) Add Abidec vitamin drops to the milk. You could even blend a single slice of banana puree into the milk and see if he notices. If not, add a slice with every bottle and after a week, add another slice - really tiny amounts.
See if you can interest him in novelty food. DS started eating a wider range of food by us allowing him to eat ultra-processed rubbish. If he even eats a Macdonald's chip, say "Oh that will make you strong and healthy!" Or a biscuit.
Try some reverse psychology, by saying very calmly, " I know you only eat ham and milk and they are both very good for you, but if you ever want to have lots of energy for running around did you know that pasta is brilliant for that. You don't have to have it, but if you want some running around energy, I'll leave some here on the table." Then leave some cooled down al dente pasta - maybe even cooked lasagne sheets cut into strips and cooled as their texture is quite a lot like ham. And you could do the same with some very basic pizza. Cut a small slice of margerita and say: This is your energy food.
Then gradually, very casually, explain what other foods do - sliced cheese (choose edam or cheddar ready sliced with a texture like ham again) to give strong bones, fruit smoothies or fruit to make his skin strong etc.
For us it was a really painfully slow process - very stressful - and we didn't manage to get DS to eat enough for many years resulting in him being quite tiny as an adult. But we never gave up and now he eats a massive range of food, more than many neurotypical people. The key thing was to offer stuff very casually about 200-400 times without expecting him to eat it (unlike the NT 10-20 times) and not to say any food was better than any other. We commented very casually (not praising) that any new food he tried would make him strong and clever and happy.