Hey stranger!! Good to hear Wally is doing well!
the leash thing should have been the best way, but if you are uneasy then he is sensing it and thinking he has to protect or there is a reason not to trust. But, as you stated another good way is for the new person to walk in totally ignore Wally , no touching or eye contact until he settles down. you can also train him to be in a sit stay when someone is entering. Treats treats and more treats
good luck... it is fixable just will take time