How do you convince a hoarder that he is hoarding things and needs to get help?

Difficult to do. It is very difficult to convince someone who hoards objects to stop. Hoarding is often a part or symptom of a more complicated psychiatric illness, like ocd. Without addressing the whole picture with help from a mental health professional, you will have no chance of helping. Hoarders often do not have insight into their own problem or the anxiety of giving up their goods is too strong. Get help.
Very carefully... Hoarding (popularized on t.V. Lately)is another form of psychiatric disease; although a benign one. Confronting the individual is problematic; and should be done with respect, love, and a plan. Collecting a lot of support will be beneficial to the hoarder.