feeding all through the night?

My baby boy is three and a half months old. He weighs nearly 16 pounds. I am feeding and feed him regurly throughout the day, but am still up feeding him 5 times a night, is this normal. I do feel abit rundown. Some people have told me that my milk must not be rich enough, but my baby seems very happy and always smiling. Everyones telling me to eiher go on formula, or express it and mix it with rusks, or baby cereal. I want abit more sleep but I do not agree with this ideas. Can anyone help me as I am so tired.