Factions is for the most experienced players, its fast paced, and usually your second buy. It is mostly prized for the content of the game and ability to get max level characters very quickly. It also has the Assassin and Ritualist professions.
Prophecies is the beginner's campaign, its the slowest and easiest but players will find themselves reaping less material rewards through out the storyline as compared to other campaigns. It does how ever have the benefit of rewarding skills as quest rewards, which is unique to this campaign only. (Outside of GW:EN)
Nightfall is a great balance of the prior two. It has what Prophecies and Factions lacked and created an almost perfect equillibrium, personally I don't think ANYTHING is wrong with the campaign. Many people disagree, I simply believe by the time Nightfall was released the GW community was already dying down, thus the reception of it wasn't taken so nicely. Nightfall has a great beginner's area, gives heroes and some of the most popular skills in the game, has a lot of quests thus good material rewards, has the most variety of PvE gameplay, and has the Dervish and Paragon professions.
GW:EN story wise will only make sense to your son if he's played Prophecies. Factions and Nightfall are only sidebars when compared to Prophecies, for it contains the majority of the lore and story to the GW universe. GW:EN is a great buy for end game content.
My final verdict: If your sons an avid gamer get him Nightfall, he'll have no trouble picking up the learning curve and the difficulty is just right. If your son hasn't really played games like this before and is more casual about it, get him Prophecies, it'll be a slow and easy experience. GW:EN would be a good follow up for his birthday or something of the like.
Might I suggest never buying your son a strategy guide or anything like that either, those are rip offs. Instead direct him to this website if he is having trouble in the game.
http://gw.gamewikis.org/wiki/Main_Page