Hi L.,
My adopted daughter was on Concerta for two years. She began the Concerta at age 5, after her teacher lovingly approached me asking to have her evaluated. We, too, struggled with the loss of appetite. I learned what works, and what does not.
During the day, only offer a SMALL amount for lunch, OR do not expect him to eat much if his school provides lunch. Feed him a BIG breakfast, and provide his favorite snacks during the day but again...in small amounts. Since they have no appetite, it is easier for them to 'force' themselves to eat a few bites if they know that is all that is required than to try to choke down a larger quantity. Then, feed good snack afterschool (PB&J with lots of J!?!) and supper. Again, small quantities is the key. If they are still hungry after, they will ask for more. But, they will not be turned off by a larger portion.
***Big help...I bought a case of Ensure Plus Chocolate, add a few tablespoons of chocolate syrup and a teaspoon or so of sugar to it, and she drinks that like crazy. By itself, she turns her nose up, but her doctor said it would not hurt to spike it up a little to make it more appealing, so we did and it works. She is no longer losing weight, but holding her own. It takes time for body and psyche to adjust to this medication, and the appetite gets a little better with time.
Oh, and do not be surprised if after a year or two, you or his teacher starts to notice the meds not working as well. It happened on our end and we changed meds. All is well again.
Hope this helps!