There is a possibility of a mass of stool stuck and when you supplement loose bm finds its way around the mass- which is why you only see the loose stools.
This is not uncommon for our kids. A good gut clean out will help you gauge exactly where's he's at and where he should be.
Probiotics are a must, use a High quality brand and rotate then at least every 30 days!

Try some magnesium citrate. Start with a low dose and gradually increase.
George's aloe Vera is wonderful for GI health taken daily.
Is he dairy free? I'd suggest working with a MAPS practitioner, unfortunately pediatric and are not well versed in the co-morbid conditions that affect our children and the treatments to help them.

Thank you all for your suggestions. Just made an appointment with a Gastroenterologist. Some of these comments scared me a bit and I want to make sure he is receiving what he needs.

It might be necessary to keep his stools loose while his colon shrinks back to a more normal size. It is stretched when stool backs up and is less sensitive to the need to go, which can lead to soiling (encopresis). Or what Jackie described.
Getting him flushed out is the first step.
Getting him on a schedule is the next step, so he is trying to go even when he doesn't 'feel' the urge.
When my son hasn't gone number two for more than 24hrs, we use lactulose in his juice to get things moving along.
Fiber & water are obviously important, but if that isn't enough, definitely try a good flushing and establishing a BM schedule. Our son's routine is sit on the toilet after breakfast, no matter what.
Can't hurt to try probiotics and see a more specialized doctor for sure.

Have your child tested for sluggish bowel syndrome. Untreated, your sons issue can lead to rectal prolapse. Currently going thru this with my 9 year old. The surgeons cannot repair the bowel hanging out of his rectum because it was caused by a behavior and not a genetic issue, so the chances for relapse is too great. You do have options start with a GI surgeon.