As the owner of a penis, I can say that both reactions could be possible in the same person depending on the circumstances. I think you could safely play it by ear. Start by not-quite-waking him up with some snuggling, and if his reaction is receptive rather than startled or sleepy-grumpy, you could work your way toward the surprise.
Basically what I'm saying: he may be asleep, but chances are his body language will still be pretty easy to read if you approach it slowly.
I think a blowjob that was, however, COMPLETELY out of the blue and without any kind of warning or sleepy foreplay, would indeed be unpleasantly startling and might get a reaction that you don't want.