No weight gain in 18 months

My 9 year old has not put on any weight for over 18 months.he eats very healthily and has a fairly good appetite but poor energy levels and suffers alot from aches and pains in particular stomach aches. His teeth are very discoloured and he does not eat junk food and cleans his teeth twice daily. He has a cold now for over a month which he cant seem to shake off and is very grumpy of late.
His brother has Cf but a sweat test showed he does not have it.
Wondering if you can shed any light on the no weight gain? awaiting to see a consultant but that could take months as the waiting list is quite long.

[Renee Radenberg]

His weight is just above the 10th percentile and his height is just below the 10th percentile for his age. He is still on the growth chart. His recommended daily calorie intake, for his age, is approximately 1700 calories. In the meantime, suggest he takes a daily children multi-vitamin and give him milkshakes (milk and ice cream blended together – add a banana or fruit) or yogurt smoothies (yogurt, milk, and frozen fruit blended together) which will give him extra calories, and protein. Hoped this helped you.