Bubblenesting is an instinctual thing. Bettas do it when they get that special bubblenesty feeling--as bettas age they tend to do it less and less. But if your boy is young and happy, you can encourage him to make a new nest by putting something on the surface for him to build his nest under, such as the cut-out center of a paper plate or styrofoam cup. Many people, including myself, condition their water with dried indian almond leaves--this makes the water resemble the water found in the betta's natural habitat and stimulates their breeding instinct, prompting them to build more nests.
Could you tell us a bit more about your betta and his home? What size tank is it, is it heated? What's your water change schedule like?