I have been told that this could be dermatitis, but haven't found a way to treat it. Is it possible that dermatitis at the tip of my penis could cause a tingling sensation in my scrotum and pain in the urethra?
Probably no disease at all: just the way sensitive skin normally looks, perhaps aggravated by irritation and augmented by anxiety. I assume you've discussed these symptoms with doctors in person. If so, and if you've been told that you have nothing wrong, then you ought to consider counseling, because you may well have to come to terms with the fact that there's nothing in particular wrong, and that's the way your penis is going to look. Once you accept that, I expect the pain will go away.
Best.
Dr. Rockoff