The immediate question would be is he keeping is genitalia clean with an anti-bacterial soap, and in particular the glans and the foreskin,
The same would go for you when you are preparing to have sex and on a daily basis keeping your vulva clear with an anti-bacteria type soap/gel.
Is he pentrating you without a condom, and the ejacuate is still running our with an odour. If he using a condom, is the same one each time. It could be that part of the condom has collapsed and trapped in the vagina, and is decomposing.
The next question would be how long is it since you started having unprotected sex?
This may sound odd, but if an old condom had split or come off during withdrawl and it had not been noticed, that could still be sitting in the vaginal tract/Bith canal and decomposing.
If that is the case you need to get to see a doctor as soon as possible because you are leaving yourself open to septicemia. Also your boyfriends ejaculate is the substance causing the odour, then he needs to see a doctor quickly as there could be issues with his prostate gland.
What is going through my mind is whether his prostate gland is losing its effieciency by not closinging down the urethra which allows urine to pass through, but not semen.
In theory the Prostate Gland is there to stop urine and ejaculate to be expelled at the same time. If the gland is not working properly then it may be that is exactly what is happening and both urea and semen are being introduced at the same time. Again in theory this should not happen, but doesn't mean it can't.
Does the ejaculate that falls our of the vaginal tract smell of Ammonia?
If that is the case it would be an indicator that there is something more serious going on. Both of you need to see a specialist in sexual conditions and symptons so that tests can be taken.
I know this sounds a bit crude, but if you could catch some of the ejaculate that flows out of yourvagina in to a small, sealed bottle about the size of a test tube; and get your partner to withdraw as soon as possibe after the initial ejacualtion. If he can capture the remaining semen which is still being pushed out through the glans of the penis, in again a test-tube type of arrangement which an be sealed that would be helpful.
Make an appointment for both of you to see a doctor at the same time and ask if he can get the ejaculate in both samples analysed as that may provide an answer.
Either way you need to see a doctor sooner rather than later.