Hello, thanks for your question on healthcaremagic .com. I don t think it is safe. As you are telling that there might be some problem on left side of his balls. So part of genitalia also may having infection. You are not sure that what it is. So do not take chance. As oral mucosa is highly vascular and infection can spread easily to your oral cavity and through it to body. So better to make him to consult doctor, take treatment for problem and when doctor suggest that oral or normal sex is safe now, then and then only go ahead. All the best. hope I have answered your query, if you still want to know something, late me know. thanks & regards, Dr. Chintan solanki