I'd go with a shoal of pygmy corys, or some otocinclus. Generally any fish you put with him need to be lower level fish since he will claim the top as his own territory. Bright coloured fish *may* be seen as rivals and attacked. Also consider any potential tank mates tendency to nip, those long flowing fins are just too much to resist for some fish.
Every betta is an individual it's hard to judge in advance how they will react to something. They may get on fine with tank mates for weeks or even months, then wake up in a bad mood one morning and ... well they're not called fighting fish for nothing.
In any case it would be advisable to have a spare 2 gallon ish tank available in case you need to separate the Betta from any new "friends".