ANXIETY OVER PLAN B

So my boyfriend and I had sex last night and the condom broke, so we got dressed pretty quick and went to the drug store and he bought me plan B. Now I took it within an hour after the incident. But I'm panicking because he's 18 and I'm 16 so if I got pregnant he'd end up going to prison, my parents may not press charges but his insane mother would. He wants to enlist and I don't want to ruin the chances of that. So I'm trying to stay calm and not stress my body out and give myself the symptoms of pregnancy but what are the chances??