SIL gave me a great recipe for corn muffins. I don't know if your DS would be interested in savoury muffins? Anyway, its really easy, and you can add whatever you want him to eat in the hope that its disguised in a yummy, cheesy muffin ... I can post it if you're interested.
Also, have you tried dips? You can make avocado dip for him to have with crackers or something (even raw veg if he's feeling adventurous). Also, you could try making chicken and vege patties yourself - if he likes nuggets he might like those?
I have been thinking about this a lot as my DD is already pretty fussy and I'd really like for her to have a good relationship with food as neither of her parents do, LOL. (I eat it too much, even when I'm not hungry, and DH hates eating.) One thing I've heard is just to keep presenting the food and you don't have to get cross if they don't eat it, just keep giving it, and if they don't want it they still need to keep it on their plate. Sometimes if kids don't ditch the food off their plate they will occasionally eat it...? Hope some of this is helpful