Op, what do you mean by junk food? If it's pizza, make your own using ciabatta, blend plenty veg with tomatoes, and cheese on top. Chicken nuggets - easy to make your own with no salt, 50/50 bread made into breadcrumbs, and you can blitz nuts or seeds to put through the breadcrumbs too. Make a big batch and freeze loads. Same with fish nuggets. Homemade burgers and meatballs - you can hide grated carrot etc in them.
Veg can be hidden in bolognese sauce, mince, soup, sauces. If you grate veg rather than chopping it, it literally melts into the meal and vanishes. After ge starts accepting these foods, then you can begin to use chopped veg rather than grated.
At this stage it doesn't matter if he knows he's eating healthily, the main thing is getting it into him.
Would he drink smoothies or fresh fruit juice? Dried fruit bars or bags of fruit flakes or raisins?
Oh, and make sure you all eat the same, rather than making separate meals for you and him.
It will get better. Ds1 used to be very fussy but he now eats 4 or 5 different veg and apples, and he loves smoothies and pear juice so it's now easy to give him plenty fruit and veg but not give the same each day.