From what I have read and understand:
With a dual cam you tend to have more problems with timing, but when timed correctly they produce "level" nock travel.
With a single cam they are easy to keep in time and they produce "straight" nock travel. This type is easier to handle for the beginning archer and is less complicated to understand.
I have owned:
Golden Eagle-dual cam
PSE-single cam
Hoyt Xtec-cam and a half
Bowtech Tribute-Binary Cam
I would recommend something in 33"-36" axle to axle, and if he is going to have the pro shop do all maintenance then cam type won't matter--get whatever he likes. If he is going to do some stuff himself like changing strings and tuning then get the best feeling single cam bow. A bow in the 35-36" range axle to axle with a brace height over 7" will be good for learning the ropes and establishing form. The main point would be to get a bow that you enjoy shooting based on vibration, hand shock, noise, and how it fits. For a beginning archer it is sometimes better to get something with an adjustable draw length to experiment with what fits best a little change at a time over several shooting sessions. Don't forget to consider shooting with heavy clothing if getting a hunting bow, as the clothing can affect the draw length.
Good luck