This is not normal.  Male dogs can have a yellow-green discharge from the prepuce called smegma--though this should not be severe enough to cause repeated staining.  These stains could be urine, pus, or gastrointestinal discharge.  Most likely it is from the urinary tract.  Your veterinarian will do a thorough examination and most likely recommend a urinalysis to start.

He had prostatitis. Thanks for the advice. He is on pills and should be fine.

Thanks a million for that. I will bring him today. Great to have your advice.