Options:
1. Call your insurer and ask if you can add a second car to your policy for which you are not the registered keeper (surely your son's friend will allow you to pickup the car for him). If they ask why you are insuring a car for which you are not the registered keeper, keep the information offered to a minimum but do not lie. Perhaps simply that you need to move the vehicle from one place to another to aid your son who is having financial troubles? If they say no, leave it.
2.
www.confused.com - or another comparison site - and check out insurance rates. They allow you to select various options for registered keeper vs you/your son as insured person. Put in the details as asked and see what they come back with. I'm sure you'll find a few. I'm sure this would be a lot cheaper for you to recover the car but that is up to you. Be sure you have one that will send you the insurance cover note by email / electronic delivery or special delivery if you're doing it in your name and want to do this before you go on holiday.
You only need a normal policy for releasing the car, don't go by the crud that you need a special policy. The car in question must be a named vehicle on the policy (it needs to show the license place on the insurance cover note unless you have dealer/trade insurance).
All policies have to give you a 14 day cooling off period... they are allowed to charge you for the time insured and an admin charge. Compare the options somewhere like confused.com gives you and see which one has the lowest charges. You should be able to get insurance to get it released and then cancelled for under 100 quid... easily.
Good job on supporting your son in this, even if he is, like you say, a f'wit
To do this though, you will have to check with the impound lot if they will release the car to someone who isn't the registered keeper. There are some car loan companies I have heard of who keep the V5 though, so I'm sure they've dealt with something a little peculiar like this in the past.
I don't think it makes much sense to part with 5 grand vs 500 to release the car, 2k to fix the car and 200-300 to pick the car up and insure it to get it back.