I think the motorized scooter might be your best option. Check out ebay, craigslist, etc. to see what they are selling for. Also, call the local scooter/wheelchair stores, explain the situation (moderate use for a short period of time), and find out if they have any reconditioned scooters they would be able to sell you. It would probably be cheaper than renting at the rates they are quoting you.
As katja pointed out, though, he is still going to need some support in walking short distances -- be that elbow crutches or a walker or something else.