Most likely the boss is some form of a threaded press fit thing, a RivNut of sorts. First thought is to do nothing and just use a hose clamp to secure the lower cage mount. Of course this means that the cage needs to have a tab like portion that can be trapped under the hose clamp. The other end of the range of options is to remove the old and fit a new one to the frame. This is usually done by some one who has the tool to press/set the boss in place as well as the ability to R&R the BB in case any old boss bits drop down in the tube during removal. I would consider using epoxy when setting in the new boss. In between these two is to try to just retighten the boss. I have found this to be far less reliable then complete replacement. A simple bolt and nut properly used can pull up the boss, lube the bolt threads first.

My vote is the hose clamp. Andy.