Sorry to have to welcome you to Purrs at such a worrying time
I know it must feel like an absolute eternity but is still early days yet, there are so many stories of cats coming home after weeks, months and years away - have everything crossed that you're not waiting much longer
I would think it very very unlikely that a fox or owl had 'got him', at 5 he's a fit and healthy youngster and being a Maine Coon am guessing he's on the larger side, you do hear of rare cases of foxes v cats but the cat is generally poorly/elderly and an 'easy target'.
There's a big long list of hints and tips on things you can do to help find him here, there might be one or two you haven't tried yet http://www.purrsinourhearts.co.uk/index.php/topic,305.0.html
An idea I had for if my cat is ever lost is to park your car for the whole day in the first space outside a big supermarket or similar and literally plaster every square inch in missing posters - people who wouldn't normally stop and look at posters on lamposts etc can't help but notice a very strange looking poster/car! Is there a supermarket or similar near where he went missing from? This would increase your target audience by hundreds if not thousands of viewers.
Another avenue to explore is facebook, I'm not on there myself but it hasn't escaped me how massive it is nowadays and how things can go viral so easily so if you're on FB get posting and ask people to share.