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Hello I will help you with your question,
With your son's high speed commute he puts more heat into the engine than you did on the around town driving so it all dumps out as he slows down and he gets the overheating from the combustion gass leaking into the cooling system theough the failing head gasket and that causes the radiator to boil out.
Replace the head gasket and you should be better. If it has been severly overheated several times the piston rings will lose their tension and it will burn oil.
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With those symptoms it is a head gasket leaking.
Best to let the "mechanic" come to their own conclusion but if they start talking about blockages and such I would move on. An independent asian shop would be my first choice.
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The combustion gasses are leaking into the coolant when it gets hot. There is no coolant leaking into the oil. The catalytic converter does a good job of buring the coolant so you don't see white smoke.
This is called spewing if you do a high speed run and the raditor empties as it gets hot at the end of the run. See it on Subarus all the time. A chemical test might show gas in the coolant but that is not a very accurate test and it will confirm gas if it changes color but I would not rule it out based on no color change.