Take him off of the psyllium immediately! This is causing his problems and here's why: psyllium swells to 50 times its size when wet - this does not sound like much until you consider that just one TEASPOON of psyllium creates OVER 1 CUP of stool. Metamucil is psyllium based too. He is simply dehyrated from the psyllium.
He needs to drink more water and fruit juices - prunes are good as are fresh fruits and vegetables. Hydration is the key for him to have more regular bowel movements.
And yes - sometimes parasite infection can cause constipation simply because of them blocking the intestines and affecting water absorption.