If you want to use this habbit then get him pullups and have him sit on a potty when he pulls it off. This may lead to pulling it off and sitting on the potty before hand.
You can try sitting your child on a potty chair several times a day to get him/her used to it. You take your child into the bathroom with you and show them your routine. That may help peak their interest.
"Toilet training in a Day" is a marvelous easy book to read. After reading the book, you make preparations for the toilet training day, and it actually happens that day!