Clutter House

This is indeed a difficult situation. It's hard to get a sense about how bad it is from your description. There are messy houses and there are clutter houses. Clutter houses are a hazard because there is likely hidden food and possibly vermin. It's difficult to verify this because of all the stuff. Your uncle is indeed attached to all the stuff and is reluctant to let go of the memories or it's potential usefulness he imagines. 

Is he of sound mind? A check up with the doctor will verify that problem. Doctors offices will often take information from relatives even if your uncle hasn't given permission for them to release information to you.

If the house is truly a hazard, a call to the health department, the landlord, or the police will force some action. Short of that, all you have is your ability to persuade. And I'm sure if you call authorities, your relationship with your uncle will be damaged.

His angry reaction implies you've lost your persuasive effectiveness for the moment. Use that as a way to limit to your intervention. Don't push that hard or you will lose future effectiveness. Try offering incentives based on things he likes more than his stuff. Try the positive rewards first. If that effort fails, you are left with your honest and gently stated discomfort with visiting him with his house in this condition. Are you the only interested family member? Sometimes things become more important when a person is lonely and isolated. Perhaps other family members could be coordinated in their persuasive influence to give him the same message. Offer to meet him somewhere else so you can visit more comfortably. He will get the message all the same, in a more positive frame. The best outcome is for him to decided his house needs to be cleaned up. That should be your goal.