You have been this boy's girlfriend for three months. You do not believe that having sex with him would feel right or morally correct. Those instincts are the "voices" that you need to listen to and respect. He needs to respect them, too. Fear of losing someone is not a good reason to have sex. You may believe that this will show him how much you are willing to sacrifice for him in order to please him or 'keep' him, but it will only communicate how desperate you are and how frightened you are of losing the relationship.
Don't forget that this relationship has gone on for only three months. I know that this short amount of time doesn't diminish how much you care for him, but sex should only enter a relationship when the partners have been involved in a long-term, committed, loving relationship where sex is welcomed responsibly by both partners. That is not the case in your situation. Explain your confusion about this matter to him and be very clear about why you do not feel right about having sex at this time. If your relationship is meant to continue and to grow more intimate and committed, it will withstand sexual temptations, fear, and distance. It's very tough to keep up relationships under these circumstances. Having sex at this time is certainly not the way to do it. If he understands and respects your feelings on this matter, without causing you guilt or fear about your future together, you will know that your relationship is worth your continuing emotional energy.