What temp Thermostat did he install? You may have a clogged line to your heater core and it may need to be flushed. Also you have a inlet valve before the heater core that allows you to generate heat before the engine is warm. These valves are notorious for leaking and failing. It is located (facing Engine) all the way to the back on the left side of the motor, low against the firewall. It will be a rubber line to a metal gooseneck that just pops into the inlet with clips that hold it in place. More than likely you will break the clips when you take it apart. You can buy the whole inlet for about $5 at any of you local part houses. Hope this helps. Good Luck!
"Wide Open 'Til You See God Then Brake!"
1971 Camaro Z28 RS
1994 Silverado 1500