Why do you think bumpy skin is evidence of a deficiency? It could be a reaction to something he's eating, but I've never heard of a rash b/c a child wasn't getting something. Is he drinking more milk and ingesting more dairy now that he's off formula? If so, maybe that's the culprit.
I do think multi vitamins are a good idea, but remember he is not in the minority when it comes to non-eating toddlers! Continue to offer him food that is nutritrious and some day he will eat more things. Also remember that toddlers eat the "food pyramid" in week not a day, and even then it can be spotty for periods of time.
My daughter loves the gummi vitamins. We get them at Costco and she likes them, but sorry I don't have any experience with the liquid.