also, multiple climax's in one day lowers the amount of sperm, that's why they say to those who have a hard time conceiving to do it on specific days, with time between so the sperm can regenerate. I wouldn't be too worried if you aren't very fertile, and it was his fifth climax, but you never know.
If you're worried then go get the plan b. But if you're not in your fertile window then you're likely fine.