If other recommendations fail, bop him on the nose. You’re not hurting him; it doesn’t have to be hard, just enough to surprise him and send the message that this isn’t acceptable behavior. Accompany with a firm “no” so that he gets what “no” actually means. his littermates, mom, and other dogs would give a warning snarl and snap in this situation - we don’t have that, so a quick bop on the nose is the next best thing
whatever action you take needs to be motivational enough for him to stop the behavior. Yelping or ignoring him might be all it takes to do the trick and if so, that’s fantastic. But if he isn’t fazed by that (my basenji wasn’t when she was a puppy), you need to up the ante in order to send the message that this behavior isn’t appropriate.