Have this lump examined by a veterinarian to find out what it is. Possible causes include trauma, foreign body, abscess/infection, cyst, papilloma (wart-like growth common in young dogs), or other type of tumor. If it looks gross it may be infected. Your veterinarian will help diagnose and appropriately treat this sore. Make sure you are keeping it as clean as possible and do not allow him to rub or scratch it.