Garen is a Juggernaut, a style of tank/fighter with limited mobility and CC, but high damage, durability, and speed. Other champions that fit this include
. Hope this helps. Skarner, Hecarim, Rammus, Volibear, and Nunu are also junglers, though Rammus, Nunu, and Volibear can also do well top lane, so if he wants to learn jungle, I highly recommend them. Also, another suggestion, have him run Ghost with the Insight mastery on all of these champions. Why use Flash with a 300 second (5 minute) cooldown to instantly gap close and then possiblty not be able to keep up when you can use Ghost on a 150 second cooldown to speed boost yourself and keep up with them afterwards? And also, if you're playing Hecarim, it'll increase your AD for the duration as well.