If he is on a feeding tube then it should be providing his vitamin requirements, have you tried gluten/lactose free milk or milkshake?
my brother also has autism (he is 23 now) when he was young his behaviour was impeccable at school but when he was home he was chaotic, my older son is similar to that. has your son always been like this? this is sometimes caused by the stress of following a school routine which is why they meltdown when they get home.
sometimes changes to routine or a new teacher can cause it, my sons school wont have dress down days because it upsets routine too much
during a school week my older son will sleep for 3 hours a night, for the rest of the night he will be manic, his diet is very dull by most kids standards, he has calmed down a bit but flares up if his excema is bad or like at the moment he is losing his baby teeth and it is really frightening him, unfortunately he has no understanding so it is difficult to reason with him.
I know its very difficult to do these types of diets, sometimes the behaviour could be as a result of lots of different things as well as a food issue, try not to worry about his eating, keep trying different things, kids pick up on your worries, keep the house happy and stressfree (easier said I know) but it makes a big difference