How much is he saying the tax bill is for? It may not be a bluff. I too got hit with a hefty bill as those who work in finance would have got staggered bonuses over the past few years- those in my company and a few friends in another were never taxed appropriately at the time. Tax man has finally caught up and we all had to pay.
I was able to provide a letter from the HM Revenue to prove the cost required and that I had paid. If he hasnt paid than it will be a bit of an issue as you only know what is owed when you dial the number on the letter suggesting you may owe tax.