You cannot go wrong with a Honda TRX300FW Fourtrax. THE most durable 4x4 ever made. It's the perfect size, has plenty of pep, is fairly nimble, requires the bare minimum of maintenance, and you can still get aftermarket parts for it. Have two in the family. I've calculated the mileage on my dad's '99 (which he bought new) and my equation comes out at over 40k. I have not done anything to it other then: oil changes, once a year spark plugs, one air filter, rear brake shoes, tires, 3 ball joints, tie rod ends, and valve adjustments. The factory air filter lasted until 2012, the tie rods until 2013, the ball joints 2014, and I only have to adjust the valve MAYBE every three years. Hell, it still has the ORIGINAL HONDA BATTERY NOW ABOUT 17 YEARS OLD!!!!!
I also have an '88 that is equally as tough. As a mechanic, I've worked on just about all makes of quads, and my personal/professional machine of choice is a Honda. Don't get me wrong, Yamaha, Suzuki and Kawasaki all make a quality machine. But, for me, Honda reliability and engineering is the way to go. I also feel they are a more comfortable quad. I would NEVER own a Polaris, and the only Arctic Cat I'd consider would be an early model with the Suzuki powerplants. I've actually got one for a resale project, a 2002 250 4x4.
As a side note; if your pop doesn't mind a foot shift model, they are tried, true, and proven. I'd take that over a belt ANY day. Just not my style, and they require no maintenance other then a periodic clutch adjustment. If he'd rather not, or can't, shift with his foot, a Honda ES equipped quad works very well. I picked up a 2004 Honda TRX250ES Recon for my mother back in June. It was a low hours unit that needed some minor repair due to neglect, and other then the older ES system that tends to hold gears when trying to downshift at slow speeds with a load on it, she loves it.