Potty training regression

My son is 8 yo diagnosed with AD/HD, PDD/NOS.  He takes Focalin XR in the morning and a Focalin tablet in the afternoon for school.  He is doing good with his school work, he has started OT, in the evenings he doesn't take meds so sometimes homework doesn't get done, but no big deal.  A big issue that has been going on with him since he was diagnosed at about age 6 is that he has been regressing with using the restroom. He was finally potty trained at age 4. He does not want to stop what he is working on or doing to go use the bathroom.  Sometimes he will not even be doing anything and will still have accidents.  Its very frustrating.  I don't know if this is a chemical imbalance/or a willful act.  Any suggestions??  We really need help with this.