It sounds like your dad is at least familiar with how to use a mouse (i.e. clicking the same link multiple times)
This is a good place to start as I have seen people try and use a mouse for the first time and it is amazing how awkward a mouse can be to a first time user. This is why I think it is much easier to use the internet on a tablet for people who didn't grow up using computers.
2013 was the year in which more people started connecting to the internet using something other than a desktop computer (smartphone, tablet, etc.)
This trend will continue as devices become more powerful and people realize how little they need/use their computer. The following ideas address the original question and should work equally well on a desktop or tablet computer (assuming the browser has access to a front facing camera).
1. Not knowing that one had to put the cursor in the text field and click
Most devices today have a built in camera on the front and eye-tracking technology is getting pretty good. It seems possible to set the focus of an input that the user is looking at. This action needs to cause as little friction as possible so it can't show/hide the on screen key board or anything like that. It would need to highlight the input with focus in such a way that the page layout doesn't shift at all. The camera could possibly look at your mouth as well and when your mouth is moving enter the text you are speaking into the highlighted field.
2. Double clicking fast
Currently when a link is tapped on an iPhone using chrome or safari a blue line is drawn across the top of the page to indicate progress. I think it would be hard to improve upon this and see no reason to do the same thing in a browser on a desktop. What you never want to do is block the page after a link is clicked because the user needs the freedom to continue scanning content and/or click a different link.