Your partner may have an issue with an extension of the visitor's visa, given that a visitor by definition is someone who is making a trip for a finite amount of time, and INZ expect them to be going to leave again to go back to their original country. Yes, some people do ask to extend their trip, but if he then applies for a partner-sponsored visa three or four months down the line, and it becomes apparent that he has been busy collecting evidence for partnership all the time, it will show up that the request for longer as a visitor was not completely truthful (and INZ can interpret that as fraud).
If you were to put in the application for a partner-sponsored temporary work visa towards the end of the life of his present visa, getting AS MUCH EVIDENCE AS POSSIBLE together - see this link
https://www.google.co.uk/#q=site:enz...tnership+proof for old threads with explanations and more ideas that may help - he would be entitled to stay on in any case. Once the application is lodged, he will be issued with an interim visa to cover the time after the visitor's visa runs out, while the application is being processed. Don't forget to keep collecting evidence while the processing is happening - INZ always ask for more just before a visa is issued, to make sure they don't grant it if someone's relationship has broken down in the meantime. Also, you will need a continuous record of evidence for the time of processing and on after that, ready for applying for residence later.