my ds (9) has adhd and is also a skinny guy (but i attribute it to his metabolism as well).
he's on a "medication vacation" for the summer as well. his appetite has exploded but he still does'nt put on much weight. we've talked to his doctors about this and all of us have come to the conclusion that it's kind of a "no-win" situation. if he's on the meds he does'nt eat as much, but if he's off he's so active he burns everything he eats right off. so, the doctors say to just make sure he's getting a good nutritional balance that maintains his health.
i try to keep stuff on hand that he likes (year round) that are high in calories but low in sugars and "bad" fats (don't want to ruin his teeth or set him up for health problems down the road). fortunatly there are some things he loves to eat that provide alot of good nutrients (he is a mexican food junkie, so we keep tortillas, refried beans and cheese on hand along with olives and tortilla chips, he loves pasta so i keep some of the non sugary brands of canned stuff-spag and meatball, ravioli...). one thing i think has helped in expanding his tastes has been having him do hot lunch at school-if he fails to eat it he goes without, and in eating it he's found that there are alot of foods he "knew" he would hate that in actuality he realy loves (mostly in the veggie categories).