The short answer is there is no fish he is *absolutely guaranteed* not to get along with. You have to be careful also of fish that will nip his long flowing fins.
The list Miss Wiggle linked is a good start. In general you want to avoid anything with bright colours that a Betta can mistake for another Betta (this includes Guppies). I have some Corydoras in my Betta's tank, he seems to get on fine with them. Just be ready to separate them if anything goes untoward.