The pull-ups are the big problem,they keep him to dry.
Initiation is the last step in training.
You can try the naked method to start with,let him run around the house for a few hours each day so that he will be able to see what is happening when he gets those feeling.
Once you can get an idea of his timing,try setting the kitchen timer for every hour or closest to his regular time and race him to the potty(he will always win), if he goes,maybe treat him to an m&m's as reward.
Once he starts showing improvement,take him to the store and have him pick out a couple of pairs of his favorite character underpants.
For starters though,you might want to get a few pair of the thicker cloth training pants and a