It sounds as though you are already doing what you need to do. The examination by, and your discussion with, his doctor is the first necessary step. If an additional work-up is needed, your doctor will suggest that and identify a facility that can provide it.
There are many things that can be done at home (and at school) to provide a boost to children with attention problems. Sometimes short attention is caused by distraction. Many of our homes are full of distractions—several people talking at once, the TV or radio playing loudly, etc. Parents can help the child whose attention fades too quickly by reducing the input of competing sounds and activities.
Make sure he is looking right at your face whenever you give him instructions, and ask him to repeat (in his own words) what you have asked him to do. Suggest this to his teacher also. She may protest (though I doubt it, since you say she is wonderful) and demand to know why she should have to repeat everything just for him. Some teachers will defend such reluctance by saying “The other kids manage to follow me with no problem.” If so, you might diffidently suggest that, since she doesn’t have time to do that, perhaps she could ask one of his friends to repeat the instructions to him. Chances are that gentle rebuke will cause her to rethink her response.
The most difficult thing for a parent of a child like yours is to be understanding and sympathetic. So much of the behavior appears to be oppositional—he’s doing it just to be difficult—or plain lazy. You will have to fight the impulse to moan, “You just won’t try. If you’d work at it, you could do it.” That impulse is strong as such children often have intellectual strengths (creativity, innovativeness) that make it almost impossible to believe them when they lament, “I just can’t do it.”
If you want to read an excellent book on this subject I recommend “A Mind at a Time” by Dr. Mel Levine.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.