You can do lots of second guessing but the best way to find out is to say something like:
"I'm going out have a drink with a couple friends... Want to join us?"
See? A casual invitation.
You don't take much risk of feeling rejected that way.
If he say yes, that's a positive sign.
If he says no thanks, he might either be busy with something else or not interested.
That's a quick way to find out.
Enjoy!