Hi.Technically, if your partner sells the property that is in his name alone and he uses that money to buy a new property which he puts into joint names with you, then he will be making a gift to you for Inheritance Tax purposes of half the cash assuming the new property is in joint names as soon as it is bought. However, there would be no immediate tax charge on you or him. So long as your partner lives for seven years after making the gift, it will fall out of his estate for Inheritance Tax purposes.Take a look here for information on Inheritance Tax taper relief and here for information on Inheritance Tax generally.
As your partner has an interest in two properties, you should check the CGT position. Take a look at HS283 here for information.I hope this helps but let me know if you have any further questions.