I would consider keeping him in the 5 gallon for good, scape it up nice for him.
This I agree with. Bettas are not true community fish imho.
Bettas are territorial fish and even if you are not seeing aggression at the moment, this is not guaranteed to stay this way.
The 5 gallon tank set up with a few nice plants, couple of nice easy care for plants, perhaps something like a couple of nice sized anubias, java ferns and java moss maybe.
Decor could be done with some driftwood arches or caves, anything like that for him to explore around and for stimulation around his tank.
And give him a good varied diet, this will help.
Would advise not adding any chemicals to the tank other than tap safe dechlorinator, I always find that good water parameters and a good daily routine of water changes does far more good to any aquatic livestock's health rather than adding meds.