I think he is far too young to understand what he is doing, so telling him "no" would be a complete waste of time. He has no control over his bowels yet and the warm water probably just relaxes him enough to have a motion. The only thing I can suggest is sitting him on a potty just prior to putting him in the tub, but he will probably not do anything anyway. Don't worry about it, it's no big deal, plenty of time to worry about it when he is older.