If you have his registration, then go to:
www.vehiclelicence.gov.uk/EvlPortalApp/index.jsp
You can then see if the car is simply untaxed, or if it's been declared SORN - using a vehicle whilst declared SORN is a �5000 fine and they can chuck you in prison.
Either way, it gives you the relevant links to grass this moron up.
However, no tax doesn't necessarily indicate no MOT or Insurance, but dealing with fraudulent insurance claims (as I do) I have to say there is generally some sort of correlation.
If you have his registration and don't mind sharing, I can tell you if he's insured or not as well.
I know people think we insurers will try and wiggle out of anything, but we do try and detect the dodgy claims and kick them out to save the genuine people out there from inflated costs - this year I have now repudiated 76 claims (equalling approx �440k). Idiots like this will eventually get caught out, but sadly it isn't until they've involved an innocent party