I've played both and here's a site that explains the differences better than I can. The differences are mostly shades of gray type things that only matter or make sense to those deeply involved in the hobby. Violence for both is about equal, I'd say.
http://rpg.stackexchange.com/questions/16504/what-are-the-major-differences-between-pathfinder-and-dd-4e
My advice would be for your son to go to a game store and spend some time with the materials for each. Flip through some books and manuals and see which world appeals to him most. Look at the characters and monsters and get a sense for what it will be like to "be" in that world. He'll likely feel a stronger attraction to one or the other and that's the one he should get. No sense getting one that you don't want to spend a lot of time with.
As for finding people to play with... D&D is the one everybody's heard of, but does he know for sure that other kids in his area are playing it or want to play it? Role playing isn't like regular board or video games in that you can just pop into someone's house and say, "Wanna play?" and play for an hour, put it away and be done with it. There's a time investment required to create your characters and worlds and keep up with the ongoing story. If other kids aren't into that kind of thing, finding players might be tough.
If not, he might want to check around some of the local game stores. Most have gaming groups for both systems. They may or may not be segregated by age, so he might find himself playing with older kids or adults. But almost every group I've even been involved with has been welcoming to all ages. It tends to be a good community.
That said, if he wants to "try out" both/either system in board game form (these are in no way the full experience of an RPG, but they are a decent introduction to the worlds of each, without the time investment required of a true RPG), there are a couple of options. These are ganes you can play through in a hour or so and then put away, so they might be more appealing if his friends aren't up for the full RPG experience.
Pathfinder has this:
http://www.amazon.com/Pathfinder-Adventure-Card-Game-Runelords/dp/1601255500/ref=pd_sim_t_13?ie=UTF8&refRID=1RNJ8CKEXWR7QADE0MSN
And D&D offers these:
http://www.amazon.com/Dungeons-Dragons-Castle-Ravenloft-Board/dp/0786955570/ref=pd_bxgy_t_text_y
http://www.amazon.com/Dungeons-Dragons-Legend-Drizzt-Board/dp/0786958731/ref=pd_bxgy_t_text_y
http://www.amazon.com/Dungeons-Dragons-Wrath-Ashardalon-Board/dp/0786955708/ref=pd_bxgy_t_text_z
(I'd go with Wrath of Ashardalon because it offers some of the experience of playing a campaign that you get in a true RPG, but in a more limited way.)
Hope this helps!