Well depending on your parenting style you could try a couple of different things. One would be to just let him sleep with you after he wakes at 11pm. A lot of parents take this route, but it has to agree with your lifestyle.
Or you could gently avoid getting him up. In both cases you should not speak to him very much, keep lights off, don't play or sing. Humming is okay, if it is in a monotone. Basically, you are avoiding stiumlating him so that he isn't waking up to hang out with you!
I also agree to quit the formula, and don't overfeed him. He is most likely waking out of habit and not because he is hungry. Keeping him on a good routine in the day time seems to help some babies. I am not big on letting a baby cry it out, but don't mind letting them fuss themselves back to sleep while I pat their back for a few minutes at a time.
If you think it could be reflux, try letting him sleep in his car seat or a bouncy seat and see if that helps.
Whatever you choose to do, hang in there!