I am noticing dry semen stains in my partners underwear, it's only happening approx once a week. is this normal or could it be a sign of infection?

Not semen. STD?: Semen doesn't leak in that way, leaving stains, unless he masturbates and ejaculates into his underwear. Sounds like urethral discharge. That's usually a sign of STD. However, STD discharge is unlikely to occur once a week, and STD chance depends on your and his sexual risks (other partners?). Your partner should see a doctor; if any STD risks, you both should be tested. Good luck!