1. From what you have described, I see no reason why you cannot file for their I-485 and have them stay.
2. He can legally stay in USA as long as USCIS receives the extension request (Form I-539)before the expiration of his current status. He can stay till the request is decided or till the time for which the extension has been requested, whichever comes first. It can take several months for the application to be decided.