My 8 year old son poops his pants and when he does use the bathroom its a log. I have tried a fiber supplement and mild child laxative. What can I do?

See doctor. He has encopresis and your doctor can work with you on a bowel treatment plan.
Retraining. Give him MiraLAX (polyethylene glycol) every night before bedtime to the point of inducing a diarrhea.Put him on the potty every morning and as he catches on cut back on the miralax (polyethylene glycol). There will be accidents along the way and it will take time. Rewards for success is also helpful.