Bettas will make bubble nests for many reasons, even a storm system going through your area can set them off. But not for all bettas.. I have one that never builds a nest, and I have another that will only build if I have a leaf (used for tannin - oak or IAL) floating in which he will build a nest under it. Also depends on the water- some water if too soft/hard/etc will make it very difficult for them to build a nest as the saliva bubbles won't hold and instead pop.
Is he in view of another betta? If not, exercise him with a mirror daily for a few minutes and see if his natural instincts of protecting his territory kicks in and he makes one. Otherwise.. not much you can do. Just enjoy him :) He may just be lazy :P