I agree with all the above posts, if he's still acting happy, eating, etc he's just fine. Also, in his little jar at the fish store he was exposed to lots of other bettas, both male and female. He probably has forgotten some of his breeding instincts since he's been away from other bettas for so long. Not that this is a bad thing, bettas aren't meant to constantly be exposed to each other in the wild. But I wouldn't be surprised if you put a mirror up to his tank for a few minutes (not something to do often, it can become stressful!) he would start building bubble nests again. Don't take any of this as solid fact, but it's just me and my experience talking here.