What happens if my son cannot afford the house and car that he will get when I die?

Your son will lose ther property to foreclosure if there is a loan on the property; if no loan, then he can sell the property and cash out.

If your son cannot afford the property, he will have to sell it.  If there is equity in the property, he will get the value of the equity.  If there is no equity, then he won't receive anything.  If loans on the properties are higher than the value of the properties, he can let the lenders foreclose on the house or repossess the car.  He is not responsible for any difference between the value and the amount of the loan.