I would write him a letter and express your concerns much the same that you have done here. Straight and to the point, without any undo emotions to cloud the letter that would allow any misinterpretations on your Uncle's part. That way, he will know exactly where you stand on the matter, and that you care for him just for him and not his money. (I wouldn't include the thoughts you have about standing by him though, that may appear as if you are "brown-nosing".)

After you have written the letter, let it be. You may or may not receive a response from him. Then just resume your normal activity of contacting him a couple of times a year and go from there. Expressing honest worries and thoughts to him, I feel, is the best approach IMO.

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A blessed thing it is for any man or woman to have a friend, one human soul whom we can trust utterly, who knows the best and worst of us, and who loves us in spite of all our faults.