Absolutely second what Ravima has to say.
Your son lives in some other unspecfied country at the moment. What are the tax laws which apply there?
He is moving to France. What are the property and tax laws there?
The donor is Irish.
It may be that there is no tax on gifts received in the country where he is now. So this might be the time to give him a gift. Before he goes to France.
But any gift to him now would use up his CAT allowance in Ireland anyway - I assume.
There are so many moving parts that neither Askaboutmoney nor Google is appropriate, other than to get some general ideas and principles such as:
Don't plan too far ahead with so many moving and changeable parts.
As a general principle, you probably should keep things simple unless there are very strong reasons for making it complex. For example, you should not buy a property in another country and rent it to him, unless there is a very compelling tax or legal reason for this.
If he is going to be living there for only 5 years, then he is probably better off renting rather than buying. This would be part of the principle of keeping things simple.
Brendan