I feel you as I had the same problem with DS.....At 12 months he fell off the bottom of the graph and I felt terrible.
My mchn was supportive enough to refer to a dietician she knew and then gave me the info I what I can do to boost him up a bit!
We bought from the chemist a tin of POLYDULE which you just pop into their milk or sprinkle it on their food. They use it alot in nursing homes and hospital to keep the oldies well nourished and help to keep weight on. Also just putting cheese, butter in everything. I thought I was being good by not giving him too much fatty foods etc but apparently I had to
She also suggested that those foods like macca's and kfc would not hurt for him to have in moderation but not to avoid it like I was....again thought I was doing the right thing!

Maybe speak with your mchn as you may meet the requirements to be referred to a dietician first and try diff foods