I am a lawyer in CT and practice in this area of the law.  Your boyfriend is walking a dangerous line as any physical contact that is sexual in nature may be construed as imporper sexual activity with a minor.  If anyone finds out that this is gong on, there may be some repercussions.  However, Some states have laws that say a 3 years difference (18 and 15) is not illegal.  I would locate a lawyer in your town and find out the specific statute as you may have nothing to worry about.  Most lawyers will meet with you for free the first time.