As far as eBay is concerned you just list it and sell it. There is no buyer or seller protection fot the sale of motor vehicles on eBay, so eBay won't be interested whatever happens.
It would be sensible to state in the listing that it's your son's car, but in practice this won't make much difference. The legal paperwork for transferring ownership will be between your son and the buyer anyway.
You need to impress on your son that the description must be truthful, and any faults should be declared. Remember that YOU will be responsible for the sale, will have to pay the eBay fees and be liable to receive negative feedback and penalties from eBay if the buyer isn't happy or your son changes his mind about selling. Also insist on a cash sale, with the buyer paying your son and your son giving a receipt to the buyer.
Personally, I'd advise against doing it. Selling on other people's behalf often leads to tears. If you son's old enough to have a car he's old enough to open an account here and sell it himself.
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