My boyfriend has a rash growing underneath his left armpit and is spreading to his right one. There no lumps or anything just kind of red, and he says it burns

Hi. . Hi. I cannot diagnose the reason for the rash in your boyfriend’s underarm. What are possible reasons? Has he started using a new soap or deodorant that he might be reacting to? Is he wearing clothes that fit tight at the underarm and are causing friction? If either of these might be the culprit, stop using the new product or try wearing clothes that are looser near the armpit. Wearing clothes that are made of natural fabric like cotton may also be beneficial as it will allow for better ventilation. Good hygiene is also important. Ensure that the arm is dried off well after washing. If his rash does not improve with these suggestions, he could have a fungal infection. Because the underarm is moist, warm and does not get much air, it is the perfect place for a fungus to grow. He may need to see his doctor to be evaluated.
I . I agree with above. It could be a fungal infection, contact dermatitis, psoriasis, eczema, folliculitis, or several other possibilities. A dermatologist should be able to help. Read more about fungal infections and contact dermatitis at the link below. I hope that helps.