I would try some solids. If he is a big boy and drinking a ton of formula then it might be time to try some oatmeal ... or fruit or vegetables. I started my boys at around 4 months. DS1 LOVED solids. He gobbled down oatmeal like it was the best thing he had ever tasted... he was ready! he went through a lot of fruits and vegis pretty quickly and we got the go ahead on some chicken when he was 6 months old.
DS2 was interested but took longer to get the whole bring the food to the back of your mouth thing.... tongue was still thrusting forward... he liked it and kept opening his mouth but it was a bit of a mess. By around 5.5 months he was eating like a champ.
Both of my boys are big boys and have always been hearty eaters!