Great Aunt Janet

Helping my 75 year old father in law with his application and we are confused. How can someone be legally onshore and apply if they only have a visitor visa. As far as I can tell, you are only supposed to be planning on staying temporarily on any of the visitor visas, and so you wouldn't have all the necessary documents to be able to do a substantive visa application. Can he apply as onshore aged contributory parent whilst he is here for the next month on his evisitor visa (UK)? If he applies as offshore, and then wants to visit whilst he is waiting, would he be turned down for a 3-6-12 month Visitors visa, because of his application that is pending? Another question - if we put his application in now, as onshore, one month before he is due to leave, will that be enough time for him to be issued with a bridging visa, so he can stay until his substantive visa is processed? thanks for any advice Janet