6 month old baby and solid feeding issues: constipation?

My almost 6 month old starting eating solids at about 4 months old. Since then he has had an on again/off again issue with constipation. I've seen the doctor for it once to find out if he had an obstruction in his intestines, took X-Rays and everything was fine. The doctor told me to switch to Soy formulas, use apple juice or prune juice, high fiber solids and make sure he's getting the right amount of fluids. Well I did just that, fed him prune juice and pureed prunes fallowed by a top off of soy formula for dinner one night. He was still constipated. So I stopped feeding him solids altogether. It's been about 3 days of nothing but Soy formula and today he has been constipated with hard pellet-like poops. I want to call the doctor, but he said to try this for a few weeks then if nothing works he'll prescribe us a baby laxative. I don't know if I can wait. My baby is in pain and I can't imagine it being good for his body to strain so hard. Any one ever had this problem? What did you do? Should I just bite the bullet and call the doc? Any advice would be great.