Concerta Question- need help | ADHD Information

My son has been on Concerta for about 2.5 months. The first month he was on the 18mg dose with no effect. We increased his dose to 36mg and the results were great while he was in school. We gave it to him everyday. The dr. said we didn't have to do that in the summer and that it might be good to give him a break. Once summer break started we were not giving him the med everyday but only on days when we thought he might need some additional focus for the day. We then moved to giving it to him every other day and we have noticed our son is very lethargic (much more severe on the days he has meds). Is this because of the change in med schedule or does the onset of the lethargy with concerta sometimes pop up? Any suggestions. 

Sorry the post is so long.

Jeremy
Medford, OR
My son has been off Concerta and then went back on after little success with Straterra. I found the Straterra made him very tired but the Concerta had the  very opposite effect in that he has boundless energy and takes a while to get to sleep at night.