You are assuming that he doesn't have any mental health impairments...
Has he released any details of his medical test results like presidential candidates normally do? The answer is no.
So what is he hiding?
There has been much speculation. But that's all we currently have. Personally I think he has a mental disorder of some kind. If he has, that would explain some of his actions and why so much of what he says is rubbish and why he sometimes contradicts what he has said earlier.
The
Wikipedia article on his health makes interesting reading, although, as with all things political on there, have an open mind and cross check the 'information'.
I'm not quoting that at the moment as I'm on a mobile and it's rather long.