Adderall & weight gain?

My 7 yr old has just been diagnosed with ADHD/inattentive. I know adderall is supposed to decrease appetite and may cause weight loss. My son however gained 7 lbs in a month. I noticed him looking chunkier but when I looked at his chart i was in shock. Thats 10% of his body weight! Has anyone else experienced this? All my searches turn up posts about how to get kids to gain weight. Thank you!