The Battle to Poop on the Potty

My son is 5.  I would please like your medical review of our projected plan.
BOTTOM LINE QUESTION: Can our plan hurt our son in any possible way?
TIMEFRAME:  Initial start date was this last weekend, Friday evening, FEB 26th.  Hoping to complete be March 28th.
CURRENT TOILET ACTIVITIES:  Josh pees in the morning, before afternoon nap, and before bedtime.  He can also pee upon request and he can pee in public restrooms with no distress and sitting on the toilet.  However, pooping has only occurred during naptime once the pull-up has come on.  He has pooped a few times at the beginning of his night sleep, but this is a rare occurance.
CURRENT PLAN:  We place him on the toilet and request both poop and pee with a significantly desirable incentive for a successful poop.  Should the poop not occur, he goes to nap with his underwear, and he has yet to poop in his underwear.  In the evening, we take him to the bathroom again and, so far, spend the rest of the evening up to bedtime sitting on the potty with the same situational parameters.  We plan to keep doing this until it works.
KEY FACTS:  The first time we tried, he backed up for 5 days and was in pain and exhibited severe behaviors in the classroom resulting from the constipation.  Additionally, at the outset of this attempt, we have retained some children's laxatives - chewable and suppositories.  We used a suppositorie on Sat. night after 3 hours in the bathroom and seeing him visibly try to push out the stool.  He expelled diarrhea and solid stool.  We crowed at his success, took him to his favorite restaurant, and he told every stranger he met that he had pooped on the potty.  He has experienced some of the success.  However, the following attempts have been unsuccessful and we are unwilling to keep using chemicals to help him.  Our plan is to use them again only if Josh backs up longer than 72 hours.  Aside from that, we will simply keep sitting on the potty and using a combined time/reward system.