try having sex with a thin latex condom. that should prolong his ejaculation and give you more time to get off. otherwise, it's going to keep happening. evidently, you have some really good vagina that's too good for his penis.
After a guy ejaculates he needs time to be able to get it up again. Don't expect it to be immediate, that's not possible and will never happen. After the first round of sex, try laying with him cuddling, and then you can begin kissing and initiating round number 2. Don't make it a bigger issue than it needs to be. If you put too much pressure on him, all he'll think about is how he is performing and probably will have a harder time getting an erection.