When can I start my son on baby food or cereal?

My son is 3 months old. He is takign his formula just fine. Never spits up. He however does not get full. I give him the recommended 4 oz every 3hrs but he is not satisfied. I have tried to give him water or his pacifier but he will scream until I give him more formula. I was wondering if I give him baby cereal or food if that would help fulfil his needs.