What contraception should I take ?

So this guy who I trusted and was with were saying if him and I were to have sex he would cum in me. I was skeptical about it cus I’m like I’m in school, I have a job, I am 21 why would you do that? I have future plans. Any who when we had sex yesterday he came in me not once but several times we had sex several times also. Today is my first day of ovulation. Do you guys think if I get a plan B that I could slow the sperm down before it reach my eggs? Will the plan B work since I’m ovulating already ?