1) make sure he doesn't have worms
2) My Amber had really bad diarrhea like that after her dose of antibiotics, and it went on after I stopped the medicine also, so it could be that
3) ask you vet about koapectate (that is what they gave Amber) or they also have anti-diarrheal medicines availale as well.
Best of luck, and as stated above, make sue he's drinking.