Naw man, never throw a guy off and tell him that his shit sucked and you don't wanna hear from it again. He will start asking all his customers if this is true and you will create a chain of shit, instead of just killing it between both of you.

Just tell him that you found another dealer with what you think is better quality weed. That's it man, he doesn't need to hear more. Just tell him that during all that time it was straight but now you've moved on.