vitamin supplements?

HI!
Well i've posted before about my extremely fussy eating 2yo. It's not getting any better, but we are perservering...:wall:
Thing is, 2 weeks ago DS had croup and he just isn't getting over it. The croup virus is gone, but he just seems to be constantly unwell lately. He has a day or 2 of looking really good and then crashes again with temps and runny nose/watery eyes and going off even the foods he usually does like.
I should point out, he's never been in danger of being under weight. His current weight is 13kgs.
If only I could get some fruit/veggies in to him his body might have a chance of fighting off these viruses. My mum has suggested there might be a syrup-form of vitamin supplements for toddlers? Has anyone heard anything about this/good or bad thoughts on it? I'm not usually one for things like supplements, but then i've always been a big believer in a balanced diet... well DS just isn't getting that. He eats NO meats, No fish, no vegetables, and only fruit he'll eat is a banana. He only gets milk in weetbix (won't drink milk or milkshakes etc).
Any thoughts on the subject??
Thanks!