Help! Ds will only wear big boy pants but won't go on the potty or toilet...

[lhd]

On Wednesday we went shopping and while at the supermarket I got ds to choose some big boy pants as a way of introducing him gently.. Was not prepared for what happened next..
He unpacked the pants and wanted to wear them - fine.. But then proceeded to have loads of accidents but refused point blank to go in a nappy.. Even for his day time nap...
I've gone along with it and managed to get to the stage where it is 50/50 accidents and successes he needs (what feels like) hours of persuasion to sit on the potty and I have to come up with new incentives each time!!!
Have managed to get him to wear pull ups at night by telling him they are night time pants and he is now fine with that - no battles (thank god!!)
He is also 50/50 on whether he wakes up dry from his daytime nap (largely depends on whether I can get him to do a wee before he goes to sleep)

Just wondered if anyone else has had this problem and any advice on what to do...