Should I be giving my 9 mo old vitamins?

My 9 mo old son took to solid food like a duck takes to water. Recently, he's been essentially weaning himself off of his formula. He prefers solid food, (and eats a lot of it), and will take the formula occasionally when he's tired. If you try to force it on him, he screams and cries. I read that he should be taking 28-32ounces a day still at his age. I'm lucky if he takes 20. He's not dropping any weight or losing vitality, but he's been sick this winter with 3 ear infections, one which perforated both his ears. The docs aren't concerned as it's been a bad winter. I know he needs his formula until he hits 12 mo, because of the vitamins, and I'm wondering if I can supplement with liquid vitamins since he is refusing to take the formula as much as he should. Also, would liquid vitamins possibly help boost his immune system?