He seems to know what he's doing, but never does everything right in any one problem. Make mistakes in two of them. Have your child find which problems are wrong, and have him find the mistakes. Let him use a calculator for facts if necessary so he can concentrate on the steps involved.
All children enjoy catching an adult in a mistake.
This activity capitalizes on the need of a child with Attention Deficit Disorder for control. You can turn an apparent weakness into a strength in this way and trigger concentrated effort on his part. This way if he gets confused you'll know it before he's done the whole assignment wrong. If you insist that every single problem be done perfectly under all circumstances, no matter how hard he's worked or no matter how much improvement he's shown, he may not want to cooperate by bringing the work to you. This gives him some control, and you may be surprised when he sets his target for accuracy higher than you would have.