If he's happy in the 5G I think you can keep him in there! Some bettas' personalities are different and don't adjust well to added space. Its good you have him in a 5G and probably don't need to upgrade him for awhile (if you want to, but of course you don't have to). I have 3 of my male bettas in 3 gallon tanks since they have had fin issues with larger sizes of tanksI have one betta, hes about 1 inch long without his tail. I bought him i na jar and transfered him into a 1/2 gallon tank. It is rectangular. (patterned ones at pet smart). then i transfered him into a 5 g tank & he was a lot happier....i think( his fins were up completly)
is the small tank fine?
lots of help welcome!!!!