Frequent discharge from penis tip.

I'm circumcised male. I have no pain or problem in urinating. I'm getting involuntary discharge from my penis at irregular times almost 4 times a day(everyday) for the past 2 years. I have consulted many urologist but couldn't get a solution to this problem. I got no Kidney stones and other issues like inflammation, infection.  I got no pain. One doctor said it was secretion from penis and it will eventually stop. But this discharge didn't stop from coming. It is not a pus or sweat which comes from elsewhere, it comes from urethral opening of my penis. My discharge is very little white in color. It is not liquidy when it comes, often it comes as little dry. Everytime when it comes it is not more than a drop in quantity. When I see my underwear it looks like one drop size polyethylene cover sticking to my underwear. It seems like it comes from inner skin of urethral opening touches my underwear. How to solve this problem of discharge?. I have attached the image of my discharge.