Hi,
Inheritance tax (IHT) may be liable upon your death if your entire estate totals in excess of the "nil rate band" currently �300,000. Anything above this is liable for tax at 40%. This is regardless of whether your son owns part of the property. The value of your estate for probate/IHT purposes is based upoin your over all wealth.
For Capital Gains tax (CGT), should your son decide to sell the property after your death, then as the property is not his main residence, then he may be liable for CGT.
Hope this helps....