suspension
You can lay that shock down too on the 05. That will soften the ride but it will also affect weight transfer and steering pressure. If he is going to ride powder, leave it in the up position and back the preload off. Set the perload on the rear to the softest position and see if it has some sag in when he sits on it. Does it have a clicker rear shock? If so set it in the soft settting and see how it feels for him. If the regular shock, open up the small screw on the bottom of the shock 1 turn from all the way closed.
Check under the hood, there should be a chart that shows this stuff also the owners manual if you have one.