Hi Notafish,
If you can't get the car taxed tomorrow and your son really wants the car back home then you could contact a local garage to collect the car on a trailer or flatbed and deliver it to you. It won't cost too much if it's local and you must have somewhere to keep the car off-road until it is legal. It's just an option if you want the car back home with you before making it legal.
Hope that helps,
Good luck.



