IME pottying training issues are the same as picky eater food issues .... the more 'concerned' the adults are around the child not doing what the adult thinks they should the more CONTROL/POWER the child feels they have and the more hoops they will try to make the adults jump through to see what they can 'get' from them ... there are very few things most children have control over in their life and what goes into their body and what comes out are TWO major ones they can control .... the reality is unless you want it to get really UGLY you cannot force a child to eat or use the toilet ... they have to be motivated by hunger to eat food they do not 'like' and they have to be motivated to use the 'potty' because it has more benefits to than going in the diaper!
Basically you need to find a way to motivate him to want to do it without him thinking you want it MORE than HE does .... you have to stop the power struggle that is being created between home and program and if the parents are not on board with it you are fighting a loosing battle cause they will consistent undo anything you are trying to do each night and every weekend
I personally do not try to train a child who is not motivated because I do not have any interest in power struggles over training ... IME when children are motivated and truly developmentally ready they will basically train themselves with very few accidents or stress on anyone's part ... sometimes it happens before they are 2 and sometimes not until they are just about to start school and sometimes anywhere in between those ages .... but it is always 'easy' and that is the way it should be!