If you're being paid, then IMHO you should simply say to the stude that he would probably get more out of the time spent on the ground if he had prepared beforehand.
Most students will turn up at some point with inadequate preparation - so I would cut them some slack occasionally.
There might be a downside to putting him off. If he votes with his feet to go elsewhere after you have "discouraged" him - and then writes to the boss of the school saying he left to take his business elsewhere due to a personality clash with you. You might then have some explaining to do.
And as we head deeper into recession, you might be grateful for having your time wasted, but at least it was time you got paid for.