Hi M.,
I just recently started giving my daughter a multi-vitamin that I get from Trader Joes and she is 2.5 years old. Our pediatrician actaully does not recommend giving them as long as we normally have a balanced healthy rounded diet... even if we have the occasional Mickey D's!
The only reason I started giving them recently is because she is battling me with foods. She will actually go days without eating and it gives me the comfort I need that she is getting her nutrients.
I also do give her V8 Fusion Juice which has 2 servings of fruits and veggies in each 8 ounces. I water that down so she is not getting excess calories give it more as a "chaser" or dessert to meals.
See what your ped suggests and if s/he has a brand they would prefer your son to take.
Good luck and blessings to you!