Yupp, the 'bettas hating big spaces' myth is a load of BS (betta sh%^ :p). What they like or even need regardless of tank size is lots of plants... I mean, they do live in flooded fields! For a bigger tank he should be fine to reach the surface (give the current is slowed). Bettas also breathe through their gills so as long as the water is oxygenated this is a non-issue.
For comfort, however I would suggest you invest in a broad-leaved plant on which he can rest near the surface. It can be real or fake. Zoo-med also sells a floating log and a little betta-hammock to serve this purpose. You can view them here: Search Results: Products