Thanks for the extra info. There are a few things that could be going on here to cause this discharge - but it is great that you had your boy neutered at a young age because this basically rules out a prostatic problem. Prostate problems are very common in older entire dogs and can range from prostatic cysts, to abscesses to even cancer and these can first manifest themselves by causing a discharge through the penis.
The most likely cause of the problem is not in fact the penis, but the prepuce - which is the skin in which the penis sits. This quite often can become infected, or even more commonly foreign bodies (ie. barley grass seeds, plant material) can become lodged in the prepuce and cause quite a severe inflammatory reaction - causing the penis to become inflammed at it's tip and causing a discharge. In some dogs a slight discharge is normal - however if you have just noticed this now and haven't before, then I am pretty sure there is something else going on here!
I really would recommend getting your boy to your local Vet clinic for a full physical exam. Even though he has been neutered and prostate disease is VERY unlikely, your Vet should still carry out a prostate exam to rule out prostatitis (inflammation of the prostate gland). He or she may also need to sedate your boy to have a good look at the inside of his prepuce to firstly take a swab to send to the lab to get a culture and sensitivity (so that they can find the bug causing the discharge and the best antibiotic to treat it) as well as look for a foreign body in the prepuce.
This would be the best way to go for your boy and I would encourage you to do this sooner than later before this problem esculates any further!!
I hope this has helped and please reply if you need more info.
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Dr M D Edwards