I agree with Shoo - even though it's just a harmless 'hey, do you want to see this with me?' a lot of guys translate that into a date, whether that's how you see it or not. If you had said "Hey, there's a bunch of us who are going to see Wicked this weekend, did you want to come along?" the guy may have not felt compelled to tell you how he thought your relationship stood - friend or date wise.
So what's the big difference? In the second scenario, it's a group thing - there's no pressure to look nice, no fear of being alone with someone you're unsure about dating, and it's just a more relaxed atmosphere. In the first scenario, it implies the two of you being alone together - this, especially to a guy who is still testing the waters to see if there is any potential that you two may date, is very freaky. (not you, the idea of going on a date alone). You'll be in a theatre together, alone, it's dark, and well..... not saying you would do this, but some girls may take things too far.
I think it could have been handled better than an instant message, but my thinking is that when you suggested going out together (even though it's just to a play), he needed to explain things now, before something happened that he didn't want.
I also agree that this friend has an issue with separating friendship from a romantic relationship.
If he's someone you still consider a good friend, why not ask him to join a group of friends for bowling, a movie night, etc? That way, he doesn't feel the pressure to go on a 'date' per se, but then he can also see the difference between these two relationships, and who knows? He may just take you up on your offer next time.