L.R.
Ask your pediatrician and also ask a pharmacist -- it is a good idea to go to a locally owned pharmacy rather than a big chain pharmacy; the pharmacists at local ones tend (in my experience) to have a much greater tendency to answer questions thoroughly and patiently.
But meanwhile, I agree with the person who said you should consider getting him evaluated for sensory issues. At the same time...many young kids do not have some kind of sensory disorder but do just dislike certain textures for a while. Have you tried offering him ONLY milk or only juice and not water? While it's great that he is not a juice addict like some young kids, he is definitely going to lack calcium, protein, vitamin D from milk and vitamin C and potassium from orange juice (many other juices are just sugar, and though it's natural sugar, our pediatrician always said that orange juice was the only one she would recommend a child drink).
It sounds as if he is at risk of lacking some essential nutrients and this is an age when he really needs them. A multi-vitamin is not necessarily going to give him the large amounts of calcium a growing child needs, either, and probably won't contain many of the minerals his body needs.
I'd consult with his pediatrician and ask for a referral to a registered dietitian or nutritionist who specializes in young children. Your son may need some tough love regarding eating (as in not catering to his tastes but offering what he needs, rather than what he prefers, until nature persuades him to eat), or he may need other help. But I'd really get to the doctor -- and don't accept anyone, even the doctor, saying "It's just a phase, let him have what he'll eat," because what you describe -- if the food pickiness is as bad as the water-only pickiness -- is not getting him what he needs to grow. Good luck and get some professional advice.