Glad to hear things seem to be improving, it must've been a really worrying few days for you all.
Bella's anal glands once got infected after being emptied at the vet's under strictly hygenic conditions (she was scooting again within 24 hours, gave them a squeeze and thick yellow stuff came out but was easily sorted with antibiotics and no problems since then). The vet said it can be common if they've been quite full that after emptying, they get inflamed and then infected, not necessarily to do with hygene.
Anyway keep us posted on his progress, poor boy...