Is the house his home or an investment? If it's his home, there will be no CGt on it. If it's an investment, there will be cgt if he sells it for more than he bought it.
If he leaves it to your son, your son will be liable for CAT on any amount over €15,000 at 33%. (if you uncle pays CGT on the sale , the CAT will be reduced by the amount of CAT paid, I think - check this out though)
If your son lives in the house as his home for 3 years before he gets a gift of it or before he inherits it, he will not have to pay any CAT on it as long as he lives in it for a further 6 years.