Both mine had that problem. My pediatrician said to mix half oz baby apple juice with half oz water. It worked for my daughter. It didn't work for my son. There were times that I actually had to use qtips to help get it out of them. Maybe try switching formula. Do you use one with iron?
A lil water not going to hurt him....
Ask your pediatrician. My son had that issue and I was told to give him warm water or mix prune juice with water in a bottle. You can try a slither of ivory soap too.