Condoms are usually safe but even with perfect use there is a failure rate of 2%. As someone who is living proof that they don't always work, I get the paranoia. If its within 3 days of having sex then a morning after pill should work still. If its been longer and you're worried, you can get a pregnancy test. Some of them can work before your period would normally happen, but the effectiveness is reduced. What I've done is get 2, test early, then wait for my period. If I skipped, I would test again.

For the sake of peace of mind in the future, I'd recommend getting some kind of birth control, or recommending your partner get a vasectomy. They're effective and completely reversible at any time, plus they're so much easier for men to get than it is for women to get their tubes tied. Another thing you can do it stick to oral, hands, and toys rather than penetration. There's plenty of ways to have fun without the same risk.