Most Helpful Guy
Since you are both 20 the relationship may or may not last, that is irrelevant. Right now, you both love each other. Just because he is attracted to another as well as to you, doesn't mean he has lost his love for you. You have something deeper than the sex he finds himself wanting with the other woman.
Since he has said he hasn't had sex with her or anything yet. You have three options, 1. You can drop him and completely destroy his trust in women, break his heart, and "potentially" destroy his trust in himself; 2. Accept it, then snoop around and be suspicious about his movements, ultimately ending the relationship, but painting yourself in a bad light and "potentially" having some of the effects from above; 3. Accept it and work with him to help him feel more attracted to you and trust him as he trusts you. You can overcome this issue together and your relationship will be stronger than ever. Tell him you love him and trust him. DON'T back out. If he missteps, maybe even give him a second chance, but provide your conditions for continuing the relationship UP-FRONT. Than show your support, offer assistance, and deepen his trust in you by being kind and loving.
It's a trial for him as much as it may be for you. Keep in mind he came to you first, not to the other woman.