Burying himself? Does it appear as though he is scratching?
He could very well have contracted a parasite such as velvet. Unfortunately this is common for a bowl-bound betta. Bettas are tropical fish and require an appropriate tropical temperature of 77F-82F to thrive. When subjected to cooler temperatures for an extended period they are under a lot o stress which lowers their immune system and makes them susceptible.
If you can confirm the cause of his behavior I would purchase an appropriate set up and medication to treat him. In some cases all that is required is good water quality, aquarium salt, and the correct temperature.
To thrive, a betta should have about a five gallon aquarium with both filtration and heat. Bowls do not cut it.
~Bettas can live 5-7 years. Unfortunately the average is probably more like 3 years because of the many misconceptions on their care.
