No if he was properly castrated then both testicles, spermatic cord & vas deferans, epididymis will all have been removed.
The only way a testicle may have been left by mistake is if it wasnt desended properly and therefore may be still inside his abdomen or iguinal canal (this is the channel where the testicle travels from the abdomen to the scrotum)
Now of course as far as i am aware cats will probably have a prostate gland (
you know i cant remember for sure nor much about that at all ashamedly..LOL) but if they do then prostate gland would produce semen as it does in humans (most of ejaculate is prostatic fluid) but of course with the absence of testicles there would be no sperm what so ever and nor should there be the drive or desire to carry out the behaviour you describe.
how long ago was this cat neutered ?? and just to check you are meaning he went to the vets for a day, had an anaesthetic and full castration carried out ??