Hi. My name is*****'m looking at your question now and will post my answer or ask for more information here in a short while.
I cannot see your previous question so I'll answer as best I can.
If your ex-wife gifts a 50% share of a flat to your son, that will be a potentially exempt transfer for Inheritance Tax purposes and a disposal for Capital Gains Tax purposes.
If your ex-wife does within seven years of making the gift, its value will form part of her estate for IHT purposes. If 3 years have passed since the gift, then the IHT charge on the gift will be reduced in line with the table here. Inheritance Tax will be payable by your ex-wife's estate, not your son, unless the estate cannot pay the IHT in which case part of the liability will fall on your son to be paid. Your ex-wife may have a taxable gain on the disposal of a 50% share of the flat depending on whether it has ever been her main home or not.
I hope this helps but let me know if you have any further questions.
The table is here. The link in my previous post works so its odd you cannot see it. Try this one too.
If the gift occurs in the current tax year, 2016/17 (6 April 2016 to 5 April 2017), then any CGT due will be payable on 31 January 2018. The gain will be the difference between what was paid for the flat by your ex-husband and what it is worth when he sells it at under-value to your son. That proportion of the gain covered by his occupation of the flat will be exempt from CGT as will that proportion of the gain for a maximum of the last 18 months of ownership when he wasn't living in it. If the flat was let, he will get letting relief. See Example 9 in HS283 here. The first £11,100 of the taxable part of the gain will be tax free. CGT is charged on residential property gains at 18% or 28% or a combination of the two rates depending on the level of the income of the individual making the gain in the tax year the gain is made.
I can't see anything dodgy from your son's point of view as he would appear to be getting a £600,000 property for £100,000. Even if the estate had to pay IHT at 40% on that and the flat had to be sold to pay it, he'd still end up with £260,000 more than he paid for it (£600,000 @60% - £100,000). After three years, the IHT starts to taper away and after seven years there is no IHT at all. Your ex-husband is not that old at 68.