A small tip: Sign up at the newsletters at Hertz, Avis etc. They send out mails and does promote some percentages off the normal price. You can get a decent rebate!
And rent now, instead of first doing it when he arrives. You can usually save 20% by paying in advance!
I went to USA last year and I was looking at the cool cars, but decided that I wanted to keep the car price down and get something a bit economical, since the cool cars like Mustang GT and Camaro SS was above $2000 for the 16 days I needed to rent.
I still wanted an american made car. That was number one, I wasn't going from Denmark to USA and then drive around in a German or Japaneese car! American cars are pretty rare in Denmark. You usually only see the old classics like Camaros, Mustangs and various cabriolet Fords when the weather is nice...
So I ended up at Hertz website because I could pre-pay and book for a group of cars at the Hertz LAX. I could either get the Cadillac STS, (I'm 29 and I still think that car looks great! I guess most Cadillac drivers are old persons) or I could get a Lincoln Towncar. I was okay with that because both cars look like some you can drive for a long time without getting tired. It was ONLY those two cars in that group. I felt safe to get something I wanted.
When I arrived at Hertz LAX after traveling 35+ hours they tried putting me up with a Cadillac DTS. The worst kind of Cadillac. It looks dull and cheap. I showed my papers stating that I either recieved the STS or the Towncar. Not that stupid DTS. I told her I didn't want the DTS. She was really nice and we talked about Copenhagen, she had visited twice, and then she tried offering me a nice Volvo or Mercedes (which runs at the same as the American Mustang class) but I declined and told her I wanted an American car .. She ended up with offering me the Camaro SS. It would cost some more, but it was a really good offer. Under half of the normal price! I only saw the lime green one in the catalogue and I declined, and we talked further.. the Corvette wasn't really an option, it's not a comfortable car to drive 5555km in..
She then told me that the Camaro SS I would have was a brand new one, 580 miles, and in black! and she offered me an even better price, because I wasn't being a dick about the situation (that's just how I'm raised).. I accepted and payed to extra price and went out to the car.. Brand new, it even had the smell.. Fuuck a cool car!

Btw I wish your dad a great trip! I landed in LAX, drove to Las Vegas, Grand Canyon, back to Vegas, then South Lake Tahoe, San Francisco and back to LA.. 5555km, the car was just GREAT, it ran about 21mpg and the coolest part about it was the Heads Up Display. I made a video at Grand Canyon, since there wasn't anything else to do because of the snow..
I have to go back and visit again, because of the snow at Grand Canyon.. that's the excuse I'm using.. the real reason why I want to go back is probably because I've never felt to welcome in any country. I really loved being there