If he has a pneumatic Governor it could be the Diaphragm. However, a new Diaphragm cost about $280 so it pays to look into other issues like Valves needing adjustment, poor compression or Injectors needing to be rebuilt first as a new IP Governor or IP timing; Diaphragm will not cure either of those.
If you think your Piston Rings might be sticking some take a look at this thread:
http://www.peachparts.com/shopforum/showthread.php?t=258757&highlight=Marvel
I did the above to my car that had continious hazy gray smoke and it cured it.