like Scott_ said, check the o-ring in the tank nozzle and the nozzle on the truck too. The one on the truck side has a dust ring that sometimes comes off and gets stuck on the tank side, then when you put on the new tank it gets in the way and will not allow the valves to open correctly.
Or it could just be something in the tank side of the valve, try another tank and see if that changes anything. Generally if there is any issues with the tanks it's usually leaking at those o-rings because they are not changed out and deteriorate causing the leak, but occasionally you'll find what i was explaining with the dust ring getting stuck on the truck side nozzle up inside it.

Other than that the only thing i can think of on these trucks that can cause starting issues is the lp converter freezing up or the little solenoid 'rattle' valve inline with the vaccuum lines at the converter, sometimes those go bad causing a start/run issue too. When you turn on the key it should vibrate.

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.