Good question! Yes, higher intensity work is the way to go!
Assuming he is looking for overall general cardiac health and to increase his speed a bit my first recommendation is to NOT use a treadmill. I will have a post coming up soon with all the details on why I believe this, so hold tight.
I also don't recommend anyone do high intensity running without a gait analysis as running on "jammed/ poor mobility" joints will eventually cause issues. This is not a matter of "if" it is a matter of time and repeated impact. Think of landing from a jump without bending your knees to absorb the shock---same idea if the joints are not working correctly. If you are in the MN area and need a gait analysis, contact me and see this link for more info.
Other options are Kettlebells (hey, I do that too!--shameless I know), sandbags, DB intervals, car pushing, BW (body weight) drills, jumping rope, etc. I prefer all of them to the treadmill.
If you have access to a HR (heart rate) monitor, that is my preferred way to set up intervals since you can take the person's actual biofeedback and use that to set up intervals instead of guessing at a time. You can also get fancy and record HR recovery times and new ones even use HRV (heart rate variability) to assess your ability to train that day. Most people can use just a basic one and even pick up a cheap one off ebay.
Hope that helps! Any comments, let me know.