actually, constipation is only when the poo is hard n baby have difficulty poo-ing. ie,he strains very hard to poo.
its not by how often he poos.
if he is doesnt have problem poo-ing i dont think it is an issue. perhaps you can dilute the FM a lil more.
also, as they grow older, they wont poo as much already. since your doc say its fine, i think dont worry too much.