After I find out about it we have big fight and 10 minutes later he doesn't remember. How can I remind him that he used to be a very intelligent person and stop doing this.

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The only thing you can do is supervise him, unfortunately like a child. If he has dementia, perhaps he should not be outside unsupervised anyway. When he heads for the trees or a wall or wherever and you see he’s about to “go”, redirect him inside. Or, encourage him to use the toilet before he goes outside. I can tell you that my husband did that once and our neighbor called the police who came to our door to question him. I was absolutely mortified.

Reminding him that “he used to be a very intelligent person” isn’t going to work, because that isn’t him now. All you will accomplish is to remind him that he’s not who he used to be and has a devastating disease. You need to accept him for who he is now and make things as easy for him as you can.
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