For how long this is happening?
Has he noticed this even when your aren't sexually engaged?
Can you describe the characteristics of this liquid (smell, consistence, color)?
I don't think this liquid has connection with the prostate gland and prostate cancer appears with other symptoms.
A prostate is a gland of the male reproductive system. It is located in front of the rectum and just below the bladder, the organ that stores urine. The prostate is quite small and wraps around a tube called the urethra, which carries urine from the bladder out through the tip of the penis.
The prostate is made up largely of muscular and glandular tissues. Its main function is to produce fluid for semen, which transports sperm. During the male orgasm (climax), muscular contractions squeeze the prostate's fluid into the urethra. Sperm, which are produced in the testicles, are also propelled into the urethra during orgasm. The sperm-containing semen leaves the penis during ejaculation.