From a numbers perspective, studies suggest that anywhere from 30 to 70 percent of men develop gynecomastia, and most of those are during puberty. Of those that develop during puberty, 95 percent of them will resolve within a 3 year period after onset. The other 5 percent will require surgery. At age 12, the condition is probably new and it's unlikely that any treatable hormonal issue will be discovered. Rather, gynecomastia at that age is relatively physiologic and may resolve on its own with some luck.
Rick Silverman