There are several different potential causes of these symptoms, and your boyfriend really should go see his primary care doctor
about this issue to help figure out what is going on.
One possibility, especially if this is just an area of redness on the penis, is that this is chafing of the skin, which can occur especially with vigorous sexual intercourse. Additionally, it is possible to have a sensitivity or allergic reaction to condoms or any other products, such as a lubricant, that were used during sex.
If the break out is more isolated and consists of specific blisters or spots, then this would be more concerning for a sexually transmitted infection. Other symptoms that might be consistent with a sexually transmitted infection would include burning with urination or discharge of pus like material from the tip of the penis.
Your boyfriend's doctor will be able to perform a physical examination and help figure out what might be going on. Simple urine tests can usually be performed to rule out most sexually transmitted infections. If this turns out to be a allergic reaction or irritated skin, then his doctor can prescribe a topical medication to relieve the symptoms and speed healing.