If it's a red patch, whatever caused it in the first place he now doubtless has a bacterial infection (maybe cause by the scratching). He should get to the doctor and at least get a series of antibiotics. The doc can do testing for various things (yeast infection, fungal infection, bacterial infection, STDs, etc.) I wouldn't put this off -- when I got cellulitis on my arm it was a VERY stubborn infection and took two rounds of antibiotics to knock down, and that was just out in the air on my arm, where there is no moisture or source of bacteria. Take it seriously, drag him to the doc if you have to
The application of coconut oil will give your husband relief. This oil has been known for soothing the rash and blocking moisture.