Finally...
I put Collin in underwear over the weekend. He knew he was going to the bathroom but he would not let us know before he actually went. So we did have some messes to clean up. He would never go when I would put him on the potty, but it never failed that he would go about 10-15 minutes later. I put him in pull-ups when we were away from home. Then this morning, before he woke up, I went in and woke him up and put him on the potty. He was not happy to be woke up and taken to the brightly lit bathroom. So he was crabby for about 5 minutes. Then I let him stand up on the stool in front of the potty and played with a squirt toy. Finally, he went! Yipee!!! Hopefully it will just get better. I made a very big deal of it. Jumped up and down and clapped and told him how proud I was of him. We'll see what happens next.