Is he running thermostats? Pressure cap good? Sure the block isn't cracked? If yes to these, the problem could be caused by ignition timing. Has it got the proper year of carb? The '49-up engine with the "stick" distributor uses a different carb than the older engine, the vacuum port draws from a different source, causing the vacuum advance system to work wrong.
How old is the engine? Sometimes the head gaskets corrode through, allowing water flow through just the front of the engine.
What car is it in? Can the hot air circulate back over the top of the rad and be drawn through again? That will usually only happen when the car is stopped. Sometimes the air can come out front under the bumper and recirculate, too. What fan is on it? The stock one moves a lot of air, replacement ones often don't.
If the car is lowered a lot, the air can't get out of the engine room.
A few things to think about, anyway.