i would ask straight away. i would also go out and offer if he needs to use the toilet at all. my dh is a tradie and says this is something that he sometimes needs and he loves it if someone offers and he doesn't have to go through the embarassment of asking.
often he will have ladies offer him cups of coffee/tea, biscuits, scones, even chicken and avocado sandwhiches one time! which he absolutely loves