Advice please:(

I'm 18 and my boyfriend is 17. Our pregnancy was very unexpected..  I'm 13 weeks currently. During the time of conception we had broken up. Although we were still sleeping with each other, I slept with one other guy (one time) as well. Now my boyfriend doesn't think he's the father even though protection was used with the other guy. What can I say or do to make my boyfriend understand that it is more than likely his baby?:(