It sounds to me as if he's /not/ trying and perhaps doesn't want the job. Or perhaps he's just unaware of how poorly he's doing.
Either way, doing it yourself doesn't help him. In such situations, I always make a person go back and do it again. You don't have to be harsh about it so as to discourage him but you can say something like, "Good job with the floors, but you need to do the tables over again." And point out exactly what he's doing wrong and needs to correct.
If he's doing as little work as he thinks he can get away with, this will show him that you won't let him get away with shoddy work. If he honestly doesn't know that he's doing a bad job this will teach him how to improve :)
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Either way, doing it yourself doesn't help him. In such situations, I always make a person go back and do it again. You don't have to be harsh about it so as to discourage him but you can say something like, "Good job with the floors, but you need to do the tables over again." And point out exactly what he's doing wrong and needs to correct.
If he's doing as little work as he thinks he can get away with, this will show him that you won't let him get away with shoddy work. If he honestly doesn't know that he's doing a bad job this will teach him how to improve :)