This app was mentioned in 8 comments, with an average of 1.25 upvotes
I'm sure just propping a phone with a stopwatch app somewhere next to the camera would be the easiest.
Edit: Quick search yeilded this app for android and this one for ios
From what you've commented why don't you try to script the segment a little ? Not so far as every word but you should try to get him to write down the point of the video. Categorize what questions are being answered and jot down some bullet points. That way, in his head he'll know he's addressed each bullet point and hopefully continue to the next topic.
You'll notice that none of the clocks offer a second hand; that would require redrawing the screen too often. Song titles typically change less than once per minute, so that's easy to do without draining the battery. Responding to taps is also super cheap.
I had a much longer response written, but then I realized that I already have a stopwatch-and-timer app installed that puts the time in the status bar icon, which means it also works with AOD. Under 10 minutes, it even updates every 15 seconds with a pie that indicates roughly how much of the minute is left or has passed. Stopwatch and Timer is available both ad-sponsored and paid. This may not be the level of precision you were hoping for, but it works!
> I spend more time watching gameplay vids than actually playing games?
I don't know how you expect us to answer this question: My advice is to get some kind of timer app for your phone, and measure the amount of time you spend doing each over the course of a week, and you'll have your answer.
Well, there is this thing called an app called a clock or date/time, 99% of the them you can set a timer for ~15 mins, etc...
https://itunes.apple.com/us/app/timer/id391564049?mt=8
https://play.google.com/store/apps/details?id=com.sportstracklive.stopwatch&hl=en
Here are one for each iOS and Google Play