Mel.. honey, as you know, im going through a very similar thing with Jesse. Lately he has gotten a little bit better, but he is very fussy. I think at the beginnning the reason he would only eat bare minimal was because he just wasnt hungry (after going so long without eating with his gastro). You know when you havent eaten anything for a while, the tiniest thing fills you up. For Jesse, what i found worked was labels. Unfortunately. Things like Wiggles yogurt and Wiggles biscuits (theyre actually pretty good ingredients wise - all natural).. giving him a smoothy in a special Mickey Mouse cup.. etc. Fruit.. he wont eat.. will only have in a smoothy.. or one of those Splats pureed fruit things because of the novelty of the packaging.. (similar to the yogurt.. he'll only eat it if he can feed himself but thats fine by me as long as hes eating). When it comes to dinner.. he hardly eats anything. Likes mash potato again because he can do it himself and its "fun". Wont touch vegies. Meat.. will eat very rarely. He wont eat lollies or chocolate.. wont touch maccas. But just by giving him what he'll eat (labels) has increased his appetite from nothing, to now eating things like toast and sandwiches (but only when i cut them into bit size pieces or cut things in different shapes to hes use to).
Like you i am giving him the vitamins though.

Sucken eyes can mean dehidration.. is he drinking enough? Whats his skin like?
Toddler formula is fine when theyre not eating.. as the vitamins in it are more important than the sugar theyre getting iykwim.

I think it may just be a stage but I would definately just see a doctor for piece of mind. Worse they can say is just feed him what he will eat.