I can't see how it would make a difference.
If he get's a basic wage each month, and get's his commission each month, then he'll pay income tax on the combined amount each month.
If he defers receiving his commission, he'll also defer paying the tax on it, but in the month where he get's the commission, he'll still pay tax on the full amount. (Just all in one go instead of in three monthly hits.)
All other things being equal it's always better to get your hands on money as soon as possible, because you have the opportunity to use it earlier.
I'm trying to think of some arrangement whereby deferring commission to every 3 months might reduce/increase PAYE or PRSI, but I can't see any way it could.
I'd take it monthly. But hang on for one of the accountants to confirm that, or tell him to give an accountant a call. This would be a simple enough question to answer.
-Rd