Puzzlement

I have a friend who really puzzles me.
He's a great guy, fun, far too impulsive, but he never lets you know what's inside.
That can be frustrating because when you ask the question (you know, how are you going? or what's going on with you? etc) you get some vague `yeah everything's good' or a change of subject. I don't expect to be told all sorts of intimate details but when you know something is going on, a struggle of some kind, but you can't put your finger on it then no communication is hard to take.
Now we've known each other for a while and been through a few tough times so it is not like I'm placing an unrealistic expectation on him.
I know as a guy it can be hard to let things out in the open, the natural instinct is to repress and ignore. I know all about it and I reckon I got pretty good at it over the years.
Let me tell you it is counter-productive.
I just don't know how to approach the situation. Do I just let him be? Should I press harder when we have a chat?
It is hard to care about someone when they won't let you.
There have been times when he has let me. These times were a fair chunk of years ago now, at least they seem that way. I can go back to one particular time that really changed my perception and without going into details (because that would be unfair) I saw the side of him that I know is there and that he seems reluctant to let out these days.
It's easy for me to say all this in a way because I have got a lot better at saying how I feel and I think it is important. I also think it is important to let the people you care about know it too - regardless of gender.
Personally, I think we're all a bit too old (!) to be pushed around by society's desperate need to label everything.