great to hear you are going to write it all down to show his doc, Don't be too worried about what he chooses to eat, children have a knack of getting all they need from what seems like junk to adults... take chicken nuggets, they often are fortified with milk, have egg to stick the coating on or in the batter, and can actually be pretty healthy if made with breast meat and are oven cooked. Bananas are packed with potassium. even ketchup has goodies in it!
Many spectrum children have funny eating, but so too do many non spectrum kids too! look at what he eats over a week or so and you will find he probably has quite a balanced diet if a bit high in carbs and fats.
if you are still concerned there are ways of hiding goodies in junk food... sweet potato makes great fries, make your own burgers with a little pure carrot in it, pasta sauce can hide puree veg too and coloured pasta can be made using spinach or tomato. ice cream can hide fruit puree in the swirls or sauce. hey I even convinced one child that sweet corn was yellow peas and broccoli was mini trees! if he does not like chewable food then puree veg or make smooth soups. if it is a colour thing then add his favourite colour food colouring to the water when boiling eggs, their shells are porous so the colour actually soaks through and colours the egg white as it cooks! ok some people will be all oh no not colourings but if it get him to eat eggs then once in a while will not hurt!