what about having an alarm (even the microwave timer?) that goes off every hour or so, and when it goes off its "toilet time!" and everyone goes to the toilet, whether anything comes out or not. You sit on the toilet for a minute (or something) and that way it becomes a familiar, normal thing - even if nothing happens.
But I think you need to keep it light too - how about blue food colouring in the toilet - and he has to make it turn green?!
Can you take him shopping so he can choose his own undies?