I talked to him today about different options. He is mainly interested because of the increased gas mileage with diesels vs gassers.
My dad was hell bent on getting a diesel for that same reason, until I showed him the differences between maintenance costs for gas vs. diesel. Not to mention repair bills. He now says he's happy with his half ton gasser.
As a GM owner, I will attest to the longevity of the truck and the comfortable ride, but I don't like the costs of suspension parts, and for some reason mine has been eating front suspension stuff like crazy. If he plans on using it for more than a fairly mild trail, I would personally stay away from the GM's purely for that reason. And I know I will probably get yelled at for saying this, but after driving one for a couple of years now in a fleet of them, I would also stay far away from the Powerstroke diesel. The service truck I drive is a 99 F450 with the well loved 7.3 liter. 88,000 miles on it, and it drives like it has 10 times that. It is extremely gutless, and the automatic transmission seems to cycle over all the gears way too often for my tastes. It probably doesn't help that it has a 12 foot steel flatbed with a 5,000 pound carry capacity crane on it, and that it weighs in at a whopping 14,000 pounds, but with all the hype that it gets, I have been extremely unimpressed. Luckily, I don't have to drive it as often anymore.
If I had to choose between the big three, I would probably go with Dodge again. My old one seemed like it needed very little in the way of maintenance, drove fairly comfortably on and off road, and wasn't too bad to look at either. However, given the opportunity, I would love to find a new gen Tundra. My supervisor has one with the TRD package, and I love the stance of it and of course being a Toyota, it has a great fit and finish on the inside as well as the out. That 5.7 doesn't have a hard time getting out of it's own way either.