I guess the reality is, you may never find out what "spooked" your dog, the key is to help him through it.
Kind of a different approach that I just thought of, but worth a shot, right? What if you had someone else walk him in the direction that he doesn't want to go in, rather than you?
I'm guessing that walks are probably turning into a stressfull situation. Whenever you want to go another direction, you probably tense up and get frustrated and perhaps a feeling of doubt before it even really starts, because you're already anticipating his reaction and his reluctance, am I right? Even if you're just experiencing a little bit of those emotions, your dog is probably picking up on it.
Whereas if you had someone new (like a friend or family member) come and take the leash and just non-challantly started walking in any which direction, would it trigger the same reaction from your dog, or would he see it as a fresh perspective and say "hey, they don't care, why should I?".
Just a thought. Sometimes it takes a village to raise a chile (I mean dog)...