His semen have to come out of you again as it does not have anywhere to go. This process can take a day or two. If you are sure it is not an infection, do kegel exercises on the toilet after sex to squeeze it out. Wear a panty liner to keep you dry and comfortable.
He can also wear a condom, or pull out before he ejaculates to help you deal with it. Many men like the feeling of pulling out and ejaculating on your body, so this is not unusual.
It is his semen coming out, not his sperm. The sperm will go up inside you're cervix, uterus and fallopian tubes, where it will wait a few days for your egg before it dies.
You can raise your behind by putting a pillow under you for 15 minutes after he ejaculates. This will give his sperm a chance to escape from his semen and go up your cervix. His semen will however always flow back out after sex. There is nowhere for it to go but out your vagina again. It is not as if there is a hole at the top of your vagina where it can go, and it does not get absorbed by your vaginal lining. It can only flow back out of your vagina.