Here's a website you can use: http://www.tennis-warehouse.com/RacquetSelector.html
Tennis warehouse also has reviews on racquets, I think that would be useful for your son's decision.
But unlike the poster above me, I suspect it would be too early for a pro level racquet. A middle budget one would be fine, because I doubt your son is good enough to notice the difference and it's harder to play with a pro level one (less margin of error, more difficult to control the ball). With tennis racquets, the price is almost always a good indicator of the quality.
In short it's a matter of taste, different racquet brands suit different playing styles and people. Personally I use Head (Speed MP) or sometimes a Wilson frame.