Try resting his flaring for a week. I had to make sure Shiloh was all healed up before attempting, and he is slowly regaining some flare movement on that one side.
Can you check closely on the supposedly injured side? flashlight, or magnifying glass usually works :) look for abnormal edges, strange coloring, excess slime coat (white), etc :)
And I know, the books always make it sound like rocket science
I'm wondering, if you could put your injured boy into the 2.5? That way he can heal without splitting open a wound on that side of the face. I have my bettas all in 3+ gallons
unless they are in quarantine for healing purposes.
I think it is up to you to choose which tank :p