Just a thought maybe when he took the empty tank off the oring that is in the valve on the tank fell out and now is in the rego valve on the truck unscrew the new tank and look to see if there is a o ring in the rego valve if so get a small pick and take it out or sometimes you can bang the rego on the counter weight and it will fall out it should be in the tank not in the rego valve. I havea call a least once a week that someone changed the tankand the truck wont start.