Sounds like he is at the "top of the therapeutic window" as Dr. Parker calls it. The dose is too high.
Tell your doc immediately. If the 40 was not working as well. Its possible that having a high protein breakfast will be enough to make a difference. V. should be good for 12 or so hours but "many don't feel it working, and therefore miss when it quits working." When V "quits in the afternoon, the ability to finish tasks is done." P125, New ADHD Medication Rules, Dr. Charles Parker.
This link gives lots of good information.
Hope these help. I am also the CL on the ADHD forum if you have any other questions.