The rash that has appeared on the penis can be due to various conditions, the first one being a yeast infection as you have rightly surmised. In that situation the nystatin cream should be able to get rid of the infection within a week of starting its use, in some cases, it can take ten days. in case the nystatin is not effective then it would need to be treated with ORAL fluconazole therapy which can be very effective as well.
The second possibility is that of a bacterial infection which can cause a red itchy rash, usually in that case the rash may be painful in nature. In this case, the exact bacteria would need to be identified with the help of a clinical examination and testing. If there is a history of unprotected sexual intercourse then an STD panel may need to be run. Again it depends on the EXACT appearance of the rash, depending on the type of infection oral antibiotics would be required.
Third, this could be an allergic reaction to some personal hygiene product which can cause this sort of rash. In that situation, every product which he uses would need to be assessed. OTC topical hydrocortisone cream along with OTC antihistamines would be very effective in that case such as OTC Claritin and OTC Zyrtec.
Right now I would advise using the nystatin and Neosporin and adding OTC antihistamines like Zyrtec or Claritin. If you can upload a picture when he comes back I will be happy to take a look at it to see which particular infection it can be. If there is no improvement after a week then he may need oral prescription antifungals.
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