Weaning him off at the earliest sign of problems would be possible if you're keeping track of how he's responding.
does sound high to me. The usual dosage
for a 20 pound dog is 1.25 to 5mg per day. He would be getting 4 times that much. That seems excessive to me, especially for such a long time. I could see working up to that much and then weaning down from it, but not starting at that dose and just keeping it going for that length of time.