i really hate it when x's use their own children to get at the parent, its pathetic, childish and the only person they punish in the e nd is the child, what a complete *!!!, ive had years of this from xp so sympathise.
its a crying shame you cant ask anyone else as this is giving him the perfect oppurtunity to blackmail you.
try calling his bluff, say, forget it, ive made other arrangements and youve just missed out on seeing your child, or if you think, he would probably say "fine", you could do what your friend suggested, present him with a bill for childcare services provided over the years, or say, fine, take the 25 pounds off the xx amount you owe me for x yeatrs childcare.
or do as janh suggests, offer to ring his department and clear the lateness with them, he might come out in a cold sweat!