Well my husband's company just switched their preferred rental car provider to Avis. I'm perusing their points options here and generally thinking aloud...
Avis has their own mileage plan called Preferred Points, but if you look at section 4 here, you'll see that if you choose airline/hotel miles, you won't earn any Avis points. Their site is frustratingly vague about what you can do with Avis points, but it seems like a lot of it is upgrades and free GPS/Car Seat/etc. Since he doesn't need those things, he'll probably opt for airline points.
They have a long list of point partners, but here are the ones that jumped out at me:
He's currently a JetBlue guy, so let's start there. 100 JetBlue points per rental day, with a maximum of 500 points. Since most people value JetBlue points at 1.7¢ each, that's $8.50 for a 5-day rental. Since they charge a $1.00 fee per day for receiving airline miles instead of Avis points, you actually only earn $3.50 worth of points for a 5 day rental.
1 point per dollar on rental (not fees and taxes) minus a $0.75 per day fee. Hubby's last 4-day rental came out to $153. Virgin points are worth 2.2¢ each so $3.37 worth of points minus $3 for the daily fee and Wow! you just earned 37¢
United is leaving the program; Delta isn't in there at all. American has a 500 point-per-rental option with a $0.75 per day fee. With American points generally being considered worth 1.7¢ each, that's $4.75 worth of points for a 5-day rental.
As for the rest, there's a ton of foreign and somewhat obscure airlines in their earnings partner list. Unless one of the foreign carriers is one that you currently accrue miles with, I probably wouldn't bother. Though, if they were a transfer partner of Chase or Amex I might consider it.
Aeroplan (Air Canada) is in Star Alliance and for them you earn 500 for an airport rental (250 for non-airport) and pay a CAD$0.75 per day fee. Aeroplan is a transfer partner with Amex, so I've used them to book United and ANA flights with Membership Rewards points. Even if you round up the value of the Aeroplan miles to 2¢ each, you're only netting around $7 worth of miles.
Starwood Preferred Guest doesn't have the daily fee that the airline partners do, but you only get 50 miles per rental day. So at the generally-accepted rate of 2.2¢ for SPG points, your 5 day rental netted $5.50.
Obviously free miles are free miles (even if it's a tiny sum) so I'm guessing he'll take the JetBlue points. But another option would be if I had some long-unused miles sitting somewhere (like Alaska) and wanted to create activity in that account to keep miles from expiring, this would be a great way to do it.
Related: my post about car rentals and insurance for people who don't own cars.