DD is 27 months and has only JUST tipped 11 kgs....i swear she has been stuck at 10kgs for a year! She is SKINNY, to the point where i have people ask me if she is sick. you can see every rib. BUT she is happy and healthy and eats like a horse!
I have a fantastic GP but when DD was about 5 month and we went in for an vacsinations i saw another GP as ours was away, she told me she liked fat babies and that my DD needed top up and to start on baby rice....i was upset but i decided to wait till my normal GP was back before i did anything, when i saw her she actually scoffed when i told her. "you have a happy healthy baby, shes just lean, dont change anything'' were her words!
Sometimes its hard not to take every measurment under the sun and analise them all, i stopped weighing DD about a year ago casue i was getting too focused on her lack of weight gain, she eats like an adult (more than me sometimes ) it just doesnt stick on her...wish i had her metabolism!

You know your son, if you can see that he is happy, eating, doing all the right things, dont let a line on a graph tell you how heavy he should be! and certainly DONT do anything you feel isnt right for him...i.e. top ups/solids. Little roly poly babies are reasuring for us as parents but some babies are just lean. if he showed other signs, like being really grumpy, sickly, crying all the time, refusing milk etc then id be worried. otherwise, be confident that you are doing a great job! which you are!I hope you see a good doc/MCHN who puts your mind at ease. hugs to you