T.S.
Have you asked him to tell you? I know it sounds silly, but he might not actually KNOW that he should tell you. If you haven't specifically had the conversation, I'd mention it to him. And if you feel comfortable, you might ask him why he isn't telling you. Every 20 minutes is a lot to be headed to the bathroom (unless you are just starting training) so it might be he doesn't really get a chance to have to go enough to mention it.
Hope this helps,
T.