DS (7) and I both take Concerta for ADHD. We BOTH deal with decreased appetite on the medication, even in the evenings when the medication has already warn off (different than on Adderal when DS's appetite would suddenly pick up right after his afternoon crash).
Some ideas/advice:
First...Try not to worry about percentiles. Remember that it's a ranking, so SOMEONE will always be the 1st percentile, and SOMEONE will be 99th. The difference between the 10th and 75th percentile could be 15lbs or 50lbs or 2lbs, as long as the next 65 out of 100 kids fall in that window. As long as he's growing, and learning, and staying healthy, it's no big deal at all. Also note, there is some research that suggests people with true ADHD (on or off of medication) are typically thinner than their neuro-typical peers. Just something to think about.
Now, eating...
Feed him real, high calorie food for BREAKFAST, before he takes his medicine in the morning. Don't limit him to typical "breakfast" type foods. DS usually warms up a plate of leftover dinner first thing in the morning, or makes a sand which, hotdog, or plate of chicken nuggets.
Try to get in a "second breakfast." It takes about an hour for me and 90 minutes for DS before the Concerta really kicks in. I usually pack him another snack to eat at before-care during their breakfast time (about an hour after that first meal at home).
Encourage him to drink milk (assuming he's not dairy free) at every meal.
Replace the empty calorie favorites with slightly heartier versions... pasta with red sauce --> pasta with homemade meat sauce... boxed mac cheese --> mac cheese with tuna and peas, or with diced chicken and broccoli.
Let him have a late night snack just before getting ready for bed (but not so heavy it'll bother his stomach or anything while he sleeps).
Talk to him ABOUT his medication and help him understand that the feeling of not being hungry is just a side-effect of the medicine. Your son is a bit older, so you may be able to come at it more directly, but with DS we talk about his medicine tricking his stomach, so he has to use his brain so he doesn't fall for the trick. When he KNOWS he should eat, then he should eat a little something even if he doesn't FEEL hungry because he actually IS hungry... he just can't tell.
Ultimately, he may just eat like a bird and be thin all his life. DS is 7, 4'3" tall and hasn't broken 50lbs yet. He's skinny... like REALLY skinny, but he's not LOSING weight, he's growing, he's learning, he's physically active and coordinated, and I've always been thin too, and we're not going to make our lives miserable trying to bulk him up while the rest of the country worries about childhood obesity.
HTH
T.