You don't need any add-on.
You can just call/text him as normal and your membership mobile mins/text allowances cover the calls/texts same as if he was still here in Ireland.
At his end he'll need cash credit or EU roaming calls/text add-ons to cover the cost of incoming/outgoing calls and outgoing texts.
Data services will not work for him while abroad.
He should look here for details:
http://www.48months.ie/membership-plans/going-abroad
In my opinion just putting some cash credit on the account is the easiest way as the EU roaming add-ons save little or nothing over cash credit.
Note that roaming support will be changing from June 13th.
See here for more details:
EU Roaming Update
Going away? Using 48 when roaming
Hope this helps.
