Bettas tend to live quite happily on their own, but should be fine with other fish.
Other non-nipping fish will be suitable.
Also, any fish that has vibrant colours, is not suitable, as the male Betta will try and attack it.
Fish with long-flowing fins are also not acceptable.
Fish with bland colours will get along with your male Betta, so chose silver/grey, black, white, cream coloured fish.
Danios get along quite well with Bettas... I have Zebra Danios in with one of my Bettas, and they live quite well together. Just look out for fin nipping, as this might occur (quite rare).
I have heard that Guppys are a bad mate for Bettas, so don't chose them.
Also, fairly slow moving fish... Fish that swim rather fast can stress out your Betta, as he can't move as fast as them.
Be careful not to overstock your tank, and that the fish are compatible in terms of water temperature, PH etc.