While my son was adjusting to Metadate (a long-acting methylphenidate, like Concerta), he lived on Yoplait Smoothies. He couldn't stand the taste of Ensure, but liked the smoothies. Someone had told me that they may not be hungry on stimulants, but they still get thirsty, so get as many calories in as possible that way. (Good advice).

In addition to the smoothies, he had milkshakes periodically. Also, I encourage him to eat as much as possible when he was hungry (generally, first thing in the morning and late at night).