My younger brother (now 30) was a very, very fussy eater as a child. Really just ate white foods - white bread (with nothing on it), plain white rice or pasta, oven chips (but not actual potatoes), a chicken breast or fillet of cod cooked without any oils or seasoning, and that was about it. Even the rice, pasta, chicken or fish would be picked over with the delicacy of a surgeon and he would end up rejecting about 60% of it for looking or smelling a bit odd. He would drink milk or water, but nothing else.
Later in life he was diagnosed as a super taster, and I have a student at my school (female) who has the same condition. She just eats plain pasta.
People were very judgy of my parents, but their three other children, including me, were brought up alongside my brother and ate more or less anything. I don´t like bacon, my elder brother doesn´t like cucumber and now she´s an adult my sister doesn´t like ice-cream, but that´s about it. My brother was just different. If my parents gave him something else, he just would not eat it. Point blank.
Now he is an adult my brother is a bit overweight, but that´s mostly due to lifestyle as he doesn´t have/make time to exercise. He loves sports and when he has time/makes the effort to play them he goes down to a healthy weight. His overall health is good although he does suffer from a lot of colds which is possibly caused by not eating vegetables or fruit. He has got a little better at eating as he got older, he is married and his wife is vegetarian and he is a very sociable person so he has managed to teach himself to eat things like margherita pizza. However anyone outside of the family would still consider him to be an incredibly fussy eater.
Your son is probably a supertaster too. Just make sure he eats enough (of anything) and gets exercise and is happy. And ignore judgy people!! Good luck.