Hi. Your son probably doesn't have gynecomastia, since both breasts are usually involved in this condition. Given your son's age and sex, the possibility of this being breast cancer is also remote, although we can't totally rule it out. Because he pierced his nipples in the past, there's also the possibility that the lump is an infection in that area.
It's probably best to wait for the biopsy results. Speculating about the diagnosis at this point can only add to your anxiety.