Do you know what kind of putter he currently has? Like Lee said, a putter is a very personal choice. It has to fit his style of putting (stance and swing path), feel (hard or soft), and more importantly, fit his eye and be easy to look at. Some people like a traditional blade style, others like the big new monstrocities that have become popular the last few years.

Some good brands:
Odyssey ($140-300) - They make a lot of different style putters, some with a soft insert in the putter face. They have all styles, from blades, to mallets, to big uglies. One of the most popular is the 2ball, or 2ball blade. I've had the 2ball blade, loved it.

Yes! (~$150-225) - This is a relatively new (3 yrs maybe?) company, which has milled C grooves in the putter face that help get the ball rolling quicker instead of skidding on the grass. Very popular in Europe and on tour. They make all kinds of styles as well.

Ping (~150-200+) - Ping has been around for decades, and they started it all in the putting world. They have some of the most classic putter styles ever. They have some newer styled stuff as well.

Scotty Cameron ($300+) - This is the holy grail of putters. The most popular putter in the world, and on the PGA tour. Craftsmanship is 2nd to none. The basic blade styles are $299, and there are all kinds of custom options you can get that can double the price (requires sending the putter back to the putter shop). I have 2 scotty camerons, love them both.

So really $300 is the max you'll spend...you'll probably be in the $150-200ish range for something he likes.