Me and my boyfriend unprotected had sex on my ovulation day and he's wondering what are the chances of me becoming pregnant from his precum?

Significant: The conception risk would be about 10 to 15%. If you do not want to conceive you should be using effective contraception, if you do want to conceive then good luck.

Possible: Most older studies have shown low levels of sperm or no sperm in pre-ejaculate with sperm not being motile (however, don't know how promptly samples were examined). However, a study published in 2011 showed that there are some individuals with a high motile sperm count in pre-ejaculate. See: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3564677/