My 6 year old son has been potty trained for over 3 years but is starting to poo his pants. Help!?

May be constipated. One of the most common reasons for pooping in his pants is the child is constipated, and has "overflow incontinence". If your child does not have a bm regularly, or is very quick in passing a bm, his large intestine may be full of poop, and he may be waiting until he can't hold it anymore. Check his stool pattern, and also find out why he is holding his bm's.
More details. For a more detailed answer, i refer you to this article from healthychildren.Org: http://www.Healthychildren.Org/english/health-issues/conditions/emotional-problems/pages/soiling-encopresis.Aspx.