Well larger spaces make bettas stressed (except when you are about to breed, then you will need a large tank) Even a jar would make a normal male betta happy. That tank should be more than sufficient. As long as he has enough space to swim, and stretch.
Well I have my two boys in a divided 10gal tank and they use every inch of it, they aren't the least bit shy or stressed. Bettas needing small tanks is definately a myth.
However, as long as you keep up on the water changes that tank looks fine Andyboy.