Two drawbacks with treadmills, IMO.

One is they don't reproduce the real difficulties (like Ghost Driver says). You have to set it to maybe +4 incline to be equivalent to wind resistance. 4%?!?! 1.5 if you want a run to be as realistic as it can be on a treadmill.

And they don't develop the mental part of keep speed up - 'cos on a treadmill the machine does the thinking for you. True, but they are good if you dont know how to pace yourself

Having said that, they're great for basic cardio - especially in the winter when snow & ice make real running awkward. He wont have the problem of winter snow and ice 

I have the problem of running in high humidity and 40 deg heat.  But I can handle running in the cold though. 
K