Take him out for ice cream! Seriously, do something he likes with him and tell him he will do good wherever he ends up.
Cornell was a reach and so is UChicago, to everyone. (but you dont have to tell him that, he already knows) He sounds like he would have enjoyed UChicago though - that is if he took to heart what he learned from you about hard work. (Now if he did not, then UChicago would have been hell for him, so maybe he was lucky to get rejected)
Either way, you should maybe try to research everything good about his safety school and highlight those. Help him visualize a plan of maximizing the benefit of going to a good school, even though it was not his dream school.
And now that he has a safety, there is always opportunities in RD. He may be interested in other schools… if you tell us what he likes, we may have some suggestions for you for RD. Get some apps out there to schools that are comparable to UChicago. You never know, Cornell might just be it - it is just as cold over there in the backwoods of Ithaca 
and it has a higher acceptance rate too