I agree with Candice's post. Vitamins are not going to increase your son's appetite. I think you posted a request about how to get him to eat better recently too, right? Offer him nutritionally rich foods (steamed veggies, lean well-cooked but not fried meats, cheeses, yogurt, fruits), whole milk, and be a good example for him. I notice that our girl loves vegetables that my husband and I both love because she sees us eating them alongside her... that is SO important! We never need to do a "separate" meal for her at this point, she eats what we eat (she's almost 2.5 now).
We give our daughter vitamins only on occasion, like when there's a cold going around (my husband and I both work in schools) or we've been traveling, etc. I have never heard that they help with appetite.
There was a study recently that said the majority of kids in the US who take vitamins don't really need them... but the kids in the US who really need them (the ones whose parents feed them fast food instead of REAL food--and there are many reasons for this, mainly socio-economical/educational so I'm not judging here) don't get them. If you are conscientious about what you feed your child, and make sure to offer a variety of nutritious foods--he probably doesn't need them.