It means he's becoming an adult. Bubble nests are territory markers for bettas. They also serve as a place for the eggs to hatch and fry to develop during the first few days.
Who am I? Well I am a Junior Member of the International Betta Congress and a breeder of white HMPKs, red dragon HMPKs, black/red dragon HMPKs, metallic multi HMPKs, blue masked HMPKs, and now multi color HMs. Soon to add black HMs. I will be showing in New Breeder class in fall of 2012.