Hi L.-
My son used to struggle so much with this problem. He was constipated on and off from the time he was 4 months old to about age 4 or 5. It got really bad with potty-training, because it would hurt to go and then he wouldn't want to go, so he would hold it, which would make it hurt more. I know how you feel seeing him have such pain with it- I HATED it. You feel so helpless. We tried numerous things. Glycerin suppositories (1/2 of one) worked sometimes, but I hated giving them to him. When he was around 5-6 months, I gave him about a tablespoon of dark Karo syrup in a couple of ounces of water and that worked ok. I don't know if you son would eat prunes or drink prune juice, but that really worked with my son until he decided he didn't like it. You can add the juice to apple juice or dilute it with water. I also realized that bananas really affected him and would make him more constipated. There are a few baby laxatives on the market also, like the Little Tummys brand, that worked for my son. Something that seemed to work well was an herbal supplement called bowel toner that had senna and licorice in it. Adults are supposed to take 2 or 3 with a meal, but I just gave him half or one in some oatmeal or something. The only thing is it tastes funny.
I work for a pediatrics office and the providers there usually rec Miralax, probably half a tsp a day with your son's weight, until he is having a soft bm every day and then slowly wean him off. You can adjust the dose 1/2 a tsp at a time every 3 or 4 days until it works if 1/2 tsp doesn't work. My best advice would be to call your dr. and keep calling until you find something that works. They have a lot of tricks up their sleeves if they realize that the old standby doesn't work for him. I definitely felt like my son's dr. just didn't get it. I wanted to show him what my son was going through, because I felt like he just thought it was a little straining, and it was more like screaming and both of us crying and nothing coming out and we were in a vicious cycle. Hang in there and use those free phone calls to your provider! :)
God bless,
J.