If you don't trust your pediatrictian, I would get a second opition... even if you just go this once. Maybe even talk to the local hospital & see if you can see a dietitian about his eating & your concerns w/ his vitamin & nutritional intake.
Most over the counter vitamins are for kids 2 or older. So, at 15 mo. I would be concerned that they would be to strong for your little guy.
Also, I beilve that there are some ready-to-serve toddler formulas out there - have you tried any of the other types/brands to see if he can handle them?
Another thing you might want to try is - have him sit with you at meal times (that is if he isn't already) & offer him what you are eating for dinner. When I first started my kids on "our" food they ate off my plate. I'd just cut up stuff really small for them and feed it to them. Mine loved potatoes, sweet potatoes, cooked carrots, bananas, pears, cooked apples, strawberries, melons, cooked peas, oatmeal, chicken (every way I cooked it), spagetti w/ meat sauce, sloppy joes (just the meat), and soft french fries. The also like snacking on ghram cackers, vanilla waffers, dry cereals, animal cookies & the baby snacks.
I wish you luck in getting him to eat a little better & also feeling more comfertable in what he is taking in.