My baby doesn't have regular bowel movements...HELP!!

I have a one month old baby and hasn't been having a bowel movement every day like he was in the beginning. About 2 weeks ago he went almost 3 days without having a bowel moevement so I called the doctor and they told me to do the rectul stimulation with a thermometer and nothing happened. Then they told me to give him 1 oz of of prune juice that night and the again in the morning and that's what made him go that afternoon once again he went back to not having a bowel movement everyday. The following week we took him for his one month check up and we told the doctor that we wanted him to have his bowel movements normal without having to give him prune juice and he suggested similac soy, (I am currently on sensitive similac) and he would actually vomit that one so I stopped and went back to sensitivee. What I did notice is the way I was preparing my sons bottles I packed the scoop, could that be why he isn't having regular bowel movements ? Older moms tell me to give him a little water ? While other people say not to. I need other moms suggestions. I don't want the nurses to tell me to add prune juice to my sons diet. I don't want him depending on that to have a bowel movement.