What Is The Pain And Lump At The Base Of Penis During An Erection?

I am asking on behalf of my son. He has pain when he achieves an erection and the pain is at the base of his penis. He has what appears to be a knot above his ball sack along the bottom portion of the vein running along the bottom of his penis. He has no problem when he is soft, but only when he is hard. There is no sign of blood or any unusual discharge. He is scared and wondering what it could be.