Another thought it may be a lymph node .got bigger,maybe he had an infection .
The pea sized lump may have been a bite and in fact it has reacted and made the rest of the area swollen, does it itch, has he been anywhere with out a shirt at athletics or similar?
I think it is actually normal as boys start going through puberty. It can happen in both breasts or just one. It is due to hormones and the ratio of testosterone to estrogen production. something like 70% of boys have this.
However, it is always best to run these things by the doctor. If it is hot and red, thats an infection. That would definately require a doctor's visit.
our peditrician will take phone calls. If yours does, just call and ask about it but most likely it is a normal hormonal thing for your boy. good luck