Well, the best thing to do would be to check to see if the TV has a sleep timer. Then you can just tell him to set the timer for a few hours ahead so incase he falls asleep, it would turn off automatically.
Other than that, the only things i can think of are actual timers that you plug in the wall before the tv, but that could be dangerous to do since your dealing with a TV.
Most televisions come equipped with a sleep feature which you can program to go off at a specified time. Most remote controls and cable networks also have the same/similar feature. If all these fail you may want to invest in a good universal remote control which is most likely to carry this feature.
Last resort, you may be looking at buying your dad a new television as a gift, which you can place in his bedroom where he can sleep comfortably when he falls asleep; and you are to have the sleep feature and possibly a feature to turn the TV on in the morning!