First off, what is the set up of the betta tank? Is it heated or filtered? How big is it, and has it been cycled? What is your water change schedule like?
Also, is there a lot of current in the tank? For one thing, bettas don't like much current usually, and for another it makes it a bit harder for them to build a nest. If the top of the water is always moving, he might just find it impossible to build one because the bubbles get broken up by the water movement.