Firstly, you should not have let him use your vehicle without written proof. He is talking a nonsense to you, it will not affect his insurance in any way. His Insurance in the states would not have covered him in the UK. You, as the registered owner of the car, are responsible for making sure it is insured. Yes, you will be held responsible. You could be fined up to £5000 and possibly receive 6-8 points or a driving ban. This is a very serious offence. You need him to put in writing that he "willfully" lied to you in advising he was insured when he wasn't, and that you were given the impression that he was insured. This should then hopefully take the onus off you.