DD is 3 in October and weighs 14kg. Shes quite tall for her age ( we are both almost 6ft) and she is "underweight" according to the MCHN but Im not worried. You have to look at genes too and see if lighter people are in his gene etc. Just because a child is abit smaller than everyone else, doesnt mean they are unhealthy and at the end of the day, thats what matters right? Maybe try some Inner Health Plus Immune Booster to boost his immune system a little. Try and get the things that he does like to eat into him in a more high calorie meal. For eg, if he likes toast, research some home made spreads that have avacado in them..

Ofcourse, when in doubt, get it checked out