Some car insurance companies have strict rules stipulating that you need to have valid MOT on your car to be insured. This may or may not be the case with your current insurer. However not all insurers will check your MOT when you make a claim. The best thing you can do is take your car for an MOT test straight away and produce all your documents at the police station afterwards. If you don't need to show the MOT certificate to the police, then don't bring it - only bring what you've been asked to bring.
If your son was driving the car without valid mot, then you could get into problems and will have to pay a fine for doing so. Otherwise you should be fine.
As far as the insurance is concerned, just put in your claim as normal and don't mention anything about the expired mot. The mot tests are computerised now anyway, so if the insurance company wants to know about it, they don't need any proof from you. A lot of people who have been in the same situation have been able to get their claims paid out, so I hope it works out for you as well.
Let me know it goes for you.
