Baby constipation??

My son was born at 32 or 34 weeks (doctors are not sure) he is now 5.5 months old. He is formula fed (enfacare 22) .. About three or glue weeks ago I started rice cereal like 3 times in a week and a week later he got constipated. We went to the doctor and she suggested we change his formala to reguline. We did.. However,  he had too many bowel movement a day for about a week so I changed him back to the enfacare 22 per doctor advice. He has been fine.. Well I started giving him peas yesterday and he is now constipated. I just called the doctor and he doctor said hat peas do not cause constipation.. So now I am sitting here wondering if maybe the formula just isn't doing him any good ?? Or could it be the peas ? 
Has anyone had a baby get constipated from PEAS? 
Also, has anyone given the baby prune juice or apple juice to a 5.5 month old baby for constipation?? 
The doctor I spoke to over the phone said it was okay to give him prune juice but I do not completely trust that doctor because she hasn't seen my son ever and probably doesn't know much of my sons history.