My 3 year old son has been potty trained twice--when he was 2 and right before he turned 3. After being happy to use the toilet for months without accidents both times, he suddenly doesn't care anymore and constantly wets his pants. I try to stay positive

Wetting pants. Has there been new changes in his life - new room or bed, absent parent, new baby, death in the family, etc? If so, address and discuss the situation with him. Try not to scold or punish him - just ignore the situation and offer him rewards and positive reinforcement for a dry day. One frequent cause maybe constipation. Monitor frquency of his bowel movements, size and girth of the stool (pellets).
Depends. If your son is waking up dry in the am try putting him on toilet and see if he goes to the bathroom normally. If not you might want to put back in pull ups for awhile. Also it would wouldn't be a bad idea to wait some more if he is waking up wet in the am. I also believe that it wouldn't be a bad idea to have your son see his dr to make sure there is no other problem perhps.