Plan B? Should I let him?

So my boyfriend plans to buy the Plan B contraceptive pill today. He doesn't quite want to use it for its "Plan B" purposes, more like a form of birth control. Well I've been feeling really guilty and too worried about pregnancy and sex, so this is gonna be our absolute last time. He wants to ejaculate inside of me because its our last time for awhile and then just have me take the plan B after. Will this method work? I have an antibiotic but a cream only for my acne, will this effect it? I used to take a low dose of birth control for my irregular periods... does that effect anything also? And he wants to ejaculate inside me more than once I know... I just don't feel comfortable relying just on this with a 15% margin of pregnancy... Lately we've been using the pull out method and it works for us, if that gives any info on our background.