Not normal but common. Usually related to prostatic stones or a low grade infection. RARELY a serious issue.
Still, worth getting checked by a urologist. DRE, PSA and maybe an Ultrasound.
If he does an U/S I suspect he will see prostate stones which are benign.
My husband is also thesame we noticed last week his semen is dark brown is it prostate cancer? And if its safe for me that still having sex.with him? Thank you