Very sound advice. I understand how tough that is to consider when you are getting, at worst, mixed signals but this is one case where "Tell him how you feel," probably isn't a good idea. I know going against all INFP instincts :)
Some of the likelier possibilities if you were to do this would be:
a) Huh? I'm in a relationship. I thought that was obvious to you.
b) I like you too, but I'm in love with GF.
c) Well - she works on Saturdays . . . we could probably hook up then.
d) Let me run this by her . . .