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Hello from JustAnswer.
Most types of skin diseases that affect the genitalia will not pose any problem if a man performs oral sex.
However, I can provide a more specific answer with additional information.
What skin disease has been diagnosed?
I had asked for more information and have not heard back.
The vast majority of skin disease that can affect genital skin would not adversely affect any man that performs oral sex. There are some diseases that are due to an infectious agent that can be transmitted via oral sex. The most common would be herpes or HPV infections, but while these affect the skin, most people would not call them skin diseases. Without knowing the specific disease, the best that can be said is that it is overwhelmingly more likely that he would not be affected. But if you would like a more specific answer, it would be necessary to know the specific disease.