Hi J.,
We've struggled with constipation with my daughter for 3 years so I feel your pain -- and your son's! We even ended up in the ER one night getting her an enema (which REALLY worked -- eeeew!)
Make sure he's getting a ton of fiber, and fluids, but NOT apple juice. I used to give it to my daughter until the pedi pointed out that apples are part of the BRAT diet that helps bind you up. Give any juice that starts with the letter 'P' like pear, plum, and of course, prune. You can also try letting him sit in a very warm bath, obviously not warm enough to burn him but as warm as is tolerable. A little rectal stimulation with the tip of a thermometer lubed up with vaseline can work, too.
I wouldn't be afraid of taking him to the doctor out of fear that you're just going to get a chemical-filled solution. Your pedi should be able to recommend a whole variety of remedies for you that don't involve medication -- although my daughter did take miralax once when nothing else worked, at a reduced dosage, and it worked great. I usually try the nurse phone line first rather than dragging an otherwise healthy kid into the germy office :)
Good luck!