K.,
I think you'll hear the same story from lots of moms. Don't stress yourself over this, he's not ready to train. My friends did the same thing, thier kids hit 18 months and they made themselves crazy trying to toilet train. I wanted until I saw some interest on my son's part. No stress, no forcing the issues, no tears, no pull-ups. Both of my boys trained at about 3 1/2, and the process from diapers to potty took a weekend.
When we realized they were in fact ready to use the toilet, we got rid of the dipaers (expect for at night for a while), put them in regular underware and off we went. In the first 24 hours every time they had an accident, the boys got themselves undressed, put their soiled clothes in the washer, went to the drawers for dry clothes, got themselves dressed again and went back to their activities. We started on Friday morning and by Monday we were done. No stress, no worries.
When you stress and worry about the training process, the kids know, they can feel it and it makes it harder for them. You have not failed at the training issues, you've done it all right, he's just not old enough and not ready to do it. He will not go to school in diapers, I promise.
Pat yourself on the back, and know that the time will shortly come when he will be ready. Until then, give yourself a break.
G.