Michael,
If you're still the legal owner, then your son has put himself as the registered keeper of the car you lent to him. If that's the case, then bailiffs are not allowed to seize your car if your son is not paying his bills. This is because your son does not have any legal rights on your car but you have. Bailiffs would normally take something of value so that they can sell it on to get their money back. However they cannot sell something that does not belong to the person in debt in the first place.
If they have assumed it was your son's car and taken it by mistake, you can get in touch with them to tell them that the car is actually yours (proof will be required). If they fail to return it you, then you can sue them for that. You can get more information on this by contacting the Citizens Advice Bureau.