Please look into GFCF diet (for Autism, celiac, leaky gut, autoimmune conditions). This saved my then 5 yo. I know, after reading through this, you will say "but he will only eat fish (full of mercury, adding to the problem), scrambled eggs and peanuts". Once on the diet, he will expand his diet! His limited diet would seem to exclude dairy and gluten, perhaps his body has already let him know that these things harm him.
Do a search and find the lowest-in-mercury fish. Then try cooking this fish with a few veggies (maybe even blending the veggies to a smooth paste and coating the fish with this, or cooking the fish on top of the blended veggies).
He may have an egg allergy, which if common. It is also common for us to crave what is bad for us. But for the time being, see about adding some of the blended veggies to the eggs. Also, coconut oil is healing, add some of this to his foods (good for adding weight, too). Try getting some different nut butters, and introduce these (since he likes peanuts, maybe getting different nuts will expand his diet, and provide different nutrients). See about a good supplement, as from Kirkman Labs, could be mixed in his nut butter?