My son is the same...We have tried everything. Bibery and corruption but he really isn't interested. He finds the work fairly easy and when pushed to do it gets most of it right. We have had tears and tantrums, shouting sessions the lot ! But as stated above I don't think he is aware of what is at stake. We don't have a great comp near us either. Have you visited the local grammar schools and taken your son with you to their open evenings? If he sees what they have to offer this might encourage him.
We took my son to see Burnham Grammar and his school arranged a day at Hershel and when he talks about secondary school he states these as the one's he wants to go to. As you say if he is getting about 80% you don't have too far to go. We are going fairly easy over the holiday and once they are back at school and back into work mode we are going to try papers (one a night) up to the exam to try and increase his understanding and speed. Are you just going for the Bucks paper as this is just VR only and the papers are sat early Oct anyway so not much longer to go...