My ds fell right off the graph at 12months and wasn't even in any percentile! He was bf till 9months then formula till 15months.
My health nurse was great and spoke to a dietician for me who suggested polydule. It's what they put in the elderlys food and drink in the nursing homes who don't or can't eat a lot or need some bulking up. It comes in a can like formula and I bought it from my local chemist. I put it in his milk. On top of that she also recommended for fats in his diet like avocados, cheese in everything, butter etc....
He never really bulked up much even trying all this and now at almost 4yrs only weighs 14.5kg but health nurse is happy with his milestones and his general well being so don't stress yourself out over it hun.
My second child a girl is exactly the same and health nurse is happy to just put it down to genetics! Bubs number 3 is overdue so I'm sure he/she is going to be the same!!!