Yes deb, you can typically spend up to $2000 per year on an ongoing basis on maintenance and repairs before it becomes financially more advantageous to trade to a NEW, all other things being equal, of course.
My sister is a retired accountant and she has always bought new cars and basically worn them out. The new car would be delegated to second car status at age 7 or 8. The second car would be used up in 14-16 years and sold for scrap or as a beater. Then the cycle would start all over.
Our family basically does the same thing; I bought my first car in 1958, and we have been a 2 car family since 1972. Since that original purchase we have only had 16 vehicles and 4 company cars.
Since cars have become fashion statements, people usually trade too often; this helps the economy and is great for those who buy used cars. But it is expensive for the original owner. However, the California lifestyle is based on change and fashion, rather than sound economics.
P.S. With a used car you can apply the same rule. A $7000 used car has a "half life" worth $3500, and 1/5 of that is $700. Most mechanics and Consumer Reports, however, would recommend that if any repair appraches 25% of the car's current value, you get rid of it.
If your son's car is now worth $5000 running and undamaged, that would be $1250 maximum to spend on any one repair. As well, any repair that exceeds the market value of the car is a no-no, although people often exceed this becuase they really love their cars.