As I'm sure you are aware there is no CGT when you sell your principal place of residence.
You may be worrying about how long he has to stay or live in it before he can sell and still declare it as his ppr. There is no explicit time period. It will be up to him to declare it or not as it is a self assessment tax. Naturally the revenue might take a different view on it if they feel it is an investment vehicle but you will have to deal with that if it happens.
You and your fiance bought the site but only he will live in it. If he does live in it and then sells it he can claim Principal Private Residence Relief but you can't unless you also live in it.
How long between completion of the house and the proposed sale?
Exactly. Six months is a bit short a period of ownership it and the Revenue could see him a property developer trying to evade tax.
If you sell so quickly after house is completed then there is VAT to consider as well under the new rules.