Advice needed.. is the baby his?

Okay so basically me and my partner had split up last summer but was still sleeping together. I had a very drunken one night stand during the same time with no protection however he didn't cum inside me. I had unprotected sex with my partner on July 20th, and then had the one night stand on 25th July. 5 days after I had the one night stand I had a positive pregnancy test (August 1st). Is it possible that it's his baby or is it more likely to be my partner's?