I'm a new letting agent and have come across what could be my first non-resident landlord. How do i determine if i class him as non-resident for tax purposes? He has advised me he will be living abroad for about six months each year.

I suggest caution here.

It is up to the landlord to convince the HMCA that they are non resident for tax purposes.

If HMCA discovers that a taxable income earner has not paid what is due they would have the right (and power) to demand payment from you. I suggest you ask HMCA for their view.

If you have it in writing from them that it is fine to send rent to an overseas landlord that would protect you but from past experience as a landlord I would say that any income from a UK property is taxable no matter where the landlord is living so I think you are being misled

Simple answer is to deduct NRL tax. Let the owner claim it back if they are exempt.
You will be liable if you get it wrong or don't have an exemption cert if they are classed as a NRL.(non resident landlord)
You will also need to make quater/annual returns to the NRL section