Try this: if he's mouthing during play stop immediately. Don't even say anything, (or just say "no" in a firm voice ) and just stop and cross your arms and look away from him. Even turn your back for a few moments.
Losing your attention and play time will let him know that the behaviour is unnaceptable and means no more play and no more fun and no more "you".
Then redirect and start over with some other activity.