My 6-month old has started to reject solids after eating them for about three weeks - any suggestions?

Has anyone experienced this before? I started feeding my son solids about three weeks ago. He was doing really well (he was exclusively fed prior to starting the solids). However, last week, I noticed a change in his bowel movements and he appeared to be constipated - he didn't have a bowel movement for four days. My pediatrician suggested I give him pear juice and water (which didn't help) and a glycerin suppository (which did). Now, he is showing absolutely no interest in solid foods and only wants to be nursed. I'm wondering if he's ociating his stomach ache and constipation with the solids - is that possible at 6 months? Should I wait a while before trying to introduce the solids again?