Hoarding is defined as the excessive collection of items that seem to have limited or zero value. When a person like your father has reached the point where he is
endangering himself, it may be difficult to correct the problem -- particularly if he does not acknowledge that his behavior is a problem at all. The primary concerns in your father's case is the risk of a fall, potential fire hazards, and unsanitary conditions.
If your father is agreeable, I'd recommend that you work with him to develop a plan for how to decrease the clutter. It may work, for example, to choose a room and dedicate a month to cleaning and organizing it. Talk to him about donating items to a local charity of his choosing, like a disabled veterans' group or homeless shelter, for example.
Given your description of your father's behavior, he may not be receptive to the notion that his house needs cleaning up, or to your offers of assistance. If he refuses to budge, you may have to lower your expectations. The most important objective is to ensure that his home is as safe and sanitary as possible. If he's living in a less than ideal environment but is otherwise safe, I don't feel this is an issue worth ruining your relationship over.
If, on the other hand, the situation has reached a point where you feel his safety is compromised and he refuses to make changes himself or with your help, I recommend that you consult his physician or a mental health provider about what to do next.