It occurs because of a transient imbalance between how much estrogen and testosterone the body produces. It generally doesn't require any type of hormonal treatment, however, and will gradually go away on its own. Only very rarely does anyone need surgery to remove the breasts. It can take a while to go away though. For most boys it lasts less than a year, but occasionally lasts for 2 to 3 years.
As you described, the breast buds can be rather sensitive and tender to touch. It is helpful to wear loose fitting undershirts, and avoid anything tight, or that puts any pressure on the nipples. He will likely need to avoid lying flat on his stomach for a while There really isn't much you can do to make them go away sooner. Boys do tend to tease each other about it, and this can be embarassing for some children. It's important to reassure boys about this, and give them some emotional support.
If a child seems to have significant emotional disturbances because of the gynecomastia, if he has other medical or endocrine problems, or if it lasts longer than 2 years, then further medical evaluation may be indicated, and you should have him revaluated by his pediatrician.