Potty Training Blues

Well, I'm trying to get serious about this whole potty training business with Gavin. He is my last one. I swear I wasn't given the patience to do it though. I am almost ready to hire someone to do it for me. My other kids were a mix of hard and easy to potty train, but were talking about girls here. Gavin is a whole different cup of, well you know.
So I wake up geared up to start my day just hanging out in the bathroom. He talks about it a lot and will occasionally try to go when he is taking a bath or changing clothes. Sometimes he even ropes Isabelle into helping out. That never turns out good though. So this morning we were doing good. He went right before his bath and then I started asking every 15 minuets if he needed to go. He started to say he did, so back to the bathroom we went. Sitting, and sitting, and sitting. At least a good 10 minuets. Nothing happens so we give up and put the "meeshka, mooshka Mickey Mouse" underware back on. (I pulled out the big guns hoping to really amp him up.) I kid you not, not 5 minuets later he is wet. We can sit for a half hour in the bathroom and nothing happens then walk out and he pees.
What am I doing wrong?? He seems ready. Sometimes he does it. Then he just stops. I guess I will just have to stop talking about it at all and let him do it on his terms.
Either that or let my 6 year old potty train him. Kylie potty trained Isabelle and did it in a day and a half and did a wonderful job. Isabelle has never wet the bed and rarely has an accident.
No her messes are usually intentional, but that is another story all together.
Im open to any suggestions....