I was just wondering what the reason could possibly be for him licking his rear all the time?

Worms are a distinct possibility.

What does the Vet say?

Not as likely as worms, but it could be his anal glands as well. Is he having any diarrhea?

I realize you "haven't been able to take him in" BUT impacted glands are extremely painful and if it's worms he could be at death's door in a very short period. You are playing with fire. If your Vet isn't open when you aren't working, find another Vet; if you can't afford Vet care, find another owner. Sorry to be harsh but that's how it is.