It's probably just a cyst. On the penis itself they are somewhat rare, usually around all one's life (one is born with them) but they don't show up until later. My husband had one on his scrotum he had for awhile and which he had removed (for me :-) when we got married. Those are more common. Tell him to tell his doctor or you will? If he's got other medical problems, they (or meds) are probably responsible for the erection problems.