My DS2 fell off the bottom of the chart for weight (he's on the 0.9th percentile for height) so the school nurse referred him to the dietician. He now gets Paediasure Plus Juice on prescription. www.nutridrinks.co.uk/Paediasure/View-all-products.html?gclid=CLbv7eqcqLICFdJvfAodvEQArA There are different drinks for different body weights, my DS2 is 6 so your son may need a different formulation.
After six weeks of one bottle a day his weight was back on the chart however the dietician views it as a long-term solution so he will be on these for a while. We now try to get him to take two bottles a day but sometimes he only manages one. Each bottle is 300 calories so it's a substantial addition to his diet but only so long as it doesn't replace food. I usually get him to have one after his breakfast and one just before he goes to bed and I give him two stars on his star chart for a whole bottle, one star for half a bottle.
My DS1 has ASD and ADHD; he is on Ritalin and has had problems with slow weight gain but wasn't a candidate for the supplement drinks because his weight is on the same percentile as his height. I got good advice from his paediatrician to give him an extra meal each day. So now both kids get supper just before they go to bed which could be strawberries, biscuits, ice cream, toast and I always give them milk. This really helps to get a few more calories into them each day.
I also let the boys get more 'junk' food than I would ideally prefer so there are always crisps and chocolate biscuits available. I do try and get them to eat fruit and to emphasise a healthy diet but at the moment I believe it's more vital to get calories of any form into them as they are both very short and slim for their ages. The dietician's advice was to feed them when they are hungry, don't make them wait for dinner in 10 minutes, let them have something to eat whenever they ask for it.
I also leave food lying around, so if they're watching TV or playing XBox then I might go in with a bowl of grapes or strawberries and just leave them in the room so they pick at them absentmindedly (curiously Jim Steinbeck used to do this to Meatloaf to get him to gain weight - leave plates of doughnuts lying around the recording studio!)