I'm a landlord and in situations like this I've tried to be reasonable where there's a justification for a tenant wanting to leave by agreeing to allow them to break the lease as long as they pay for the rent up to the point at which the new tenant moves in. I'd be surprised if you are much out of pocket as long as he agrees to that plus pay any fees associated with marketing the property again, plus of course you can claim any owed rent from the amount held by the deposit scheme. I rent my property at a reasonable rate in a major city, though, so YMMV in regards to how easily you can find a new tenant.
Thanks Mrs. Yes, I want to be reasonable. I'm in a city too so fingers crossed it will go quickly but it's stressful and already cost me money at advertise which is in short supply at this time of year. I need to get it on the market but I'm in between jobs so don't want to do too much until I know what's happening with work. I'm a very reluctant landlord.
Just for clarity though, of course he owes you the full amount of rent up to the end of the six month lease and if he were a uk resident you could use money claim online to sue him for any rent owed, but as he is leaving the country I'd suggest trying to reach an amicable arrangement as any legal action is going to be very problematic.
joining the chorus - once he's out of the country he's out of your reach. Ask him to do the decent thing by helping you to get a new tenant in ASAP, by keeping the place immaculate and allowing viewings. He doesn't have to do either. He does have to pay the six months rent but good luck enforcing that once he's gone; so best bet is to work with him to get someone else in.
that said, I had a tenant who left early and left the country, and DID pay all he owed - we got someone else in as soon as possible to release him early. So all may be well.