Thanks for asking your question on Just Answer. Preputial discharge (discharge coming from the penis) can cause your dog to lick at his penis. Generally speaking, discharge consists of blood, pus or urine. A small amount of whitish discharge called "smegma" is normal. Disorders of the bladder, urethra or ureters (a urinary tract infection, bladder stones, etc), disorders of the prostate or urinary incontinence can cause abnormal discharge.
Here are some signs to watch for:
Swelling, spotting, lethargy, decrease in energy or appetite, fever or excessive licking.
If you see anything other than a small amount of creXXXXX XXXXX discharge (smegma), it should be checked out by your veterinarian.
I hope this helps. Please let me know if you need more information.