Alliegid - my son is now 19 but from the age of 16 to 18 he was like a stick, never having the appetite like other boys of his age - then suddenly when he turned 19 he started to 'fill out' - I think all youngsters go through body changes - in fact when my son was around 12, 13, 14 he was really quite chubby.
So I can understand your worry and if you feel the need, then speak to your GP or practice nurse.
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Sqaddie - stop telling people its no big deal when they ask a medical question, I know this is the doctor side of you speaking, but it is a 'big deal' to the person asking the question, otherwise they wouldn't be asking {:o)