Yes, he can drive the car. He'll need the original Log Book / Registration Document or whatever else they call it these days, plus an original letter from you authorisung him to take the vehicle to Turkey. If you can get it notarised in the UK, even better.
The other alternative is, if you have the time, complete the change of Registered Keeper details so that the vehicle's registered in his name and send the document to the DVLA. The current turn-round time for this is 4 - 6 weeks.