My suggestion:
Recall a few of the moments when you've been particularily unhappy with his performance. Preferably in different areas. Perhaps planning, handling, communication, attitude etc.
Why were you annoyed? - find the problem
What did he do? - narrow it down to the core
Were there any risks involved and if so did they grow due to his actions or behaviour? - produce motivation
Are there different methods to go about the same problem that would have been better (here you must look att all different ways to do something, you way might not be the best)? - look at all the loopholes
How did you behave? - would he have behaved differently if you'd done it any other way?
Ask yourself a couple of questions like this and write it down.
Then take your time to have proper sitdown with him, on ground in a relaxed area (quiet area of a hotel lounge after having changed into civil clothes). Get him comfortable and relaxed. Start asking him what he thinks is a good approach to things, use anecdotes from your previous career as a basis for discussion. Keep control of the conversation if he tries to stray off the topic. As he softens up, ask him if there are any situations between him and you that he feels could have resulted in something better. When he's had his say, ask him about the situations you've written and ask questions like "were you really hapy with that approach?" or "would you've done it differently today?"
The trick is to confront him in such a way that he doesn't understand that he's being confronted and ask the questions so that he's forced to look at himself and give himself critisism. It's tough to take critique and it's really difficult to give critique. You have to prepare him for it for him to be able to swallow some of the tasty bits you have in store for him. And if you have thought about a few situations on forehand and written them down, you'll have the upper hand.
I've used a similar type of confrontation when I want to take up things with my captains that I haven't been happy with and it tends to work. It bluntens the edge of the sharp critique sword so to say...
Best of luck.
/LnS