Inheritance Tax has to be paid, at the rate of 40%, on that part of an estate which is above the nil rate threshold. That threshold is currently £325,000.
However if your father's wife has pre-deceased him, leaving all of her estate to him, then he effectively inherits her nil rate allowance too, meaning that Inheritance Tax will only have to be paid on the the value of his estate which exceeds £650k.
It won't matter as to whether the will leaves the estate to one person, two people or lots of people. It's the estate that has to pay Inheritance Tax, rather than any individual beneficiaries. So the tax, if any, is deducted before the estate passes to the beneficiaries.
Am confused...if your not the beneficiary why might you have to pay IHT. Its the estate what pays it. Or are you saying his son gets property and money and you get something else (car, jewels, ) ????