Bf is 45 has a nickel sized raised very round hard bump under the skin in his armpit very painful has apt tomorrow but we are scared what could it be?

Possible abscess: This is a common location to have an abscess, a local skin infection, which is easily treated by opening it and allowing it to drain, which may hurt briefly, but will release the pressure and there will be less pain afterwards as it drains.. Other causes are possible of course, but that seems most likely. The doctor will sort if out, but it is probably nothing major, depending on your BF's health,