Some of them just don't seem to take any account of that type of build. Obviously it is possible to be very slim and unhealthy, but only if that's not your natural shape. The BBC version here is a bit more positive - "It could be that you eat a varied and well-balanced diet but are naturally very slim. If so, there is no need to change what you eat, but do make sure you are getting all the energy and nutrients you need to stay healthy and maintain your energy levels." Though it does then warn anyone who is restricting their food intake or using vomiting/diuretics/laxatives to get advice.
DS is also very tall and skinny (13.90!) but I can remember my brother at a similar age, and have photos of my father a bit older, and they were just the same build. Both filled out by mid-twenties. So it's just nature - if a website told DS he was unhealthy at that weight and made him try to put on weight by shovelling in junk food, I'd be very angry at them!
He DOES like biscuits, but is a healthy eater overall.
He really wants to build up his muscles, but hasn't had his growth spurt yet (is 13). Being an August birthday also makes it worse sometimes when surrounded by people a year older who have acne & shave as well as being 6' tall! He is not very tall, but IS growing
I don't think he'll be able to put on much muscle at 13, specially if he hasn't got his height yet, nothing much for it but patience I suspect . But anyway, the important thing is for him not to feel negative about it - I'm at kids websites telling him he's unhealthy, just for being his natural self! Not the right kind of attitude to give a teenager.
Luckily my DS is fine about being "slim", and we've always tried to describe it in positive ways, since it was obvious he was going to be that shape from about 18 months!
Milliways - I love seeing that pic on your profile... because it looks like my DH when he was young (he was also in 28s in his 30s...)
We are all different shapes and sizes- some people are just meant to be very thin! DH has always had an unbelievable appetite- it really isn't through lack of food intake. He took part in a study once for someone's physical therapy qualification, and his body fat was the lowest the assessor had ever seen recorded
Just show him you love him how he is- and it really helps if his dad is the same shape, because he obviously found a woman to love him
DH grew upwards at 13- was 6ft at that age, but didn't put on serious muscle until his 20s- he went to the gym from around 14, but it didn't do much physique-wise, though he became strong IYSWIM. He continued to grow until 21 too- is now 6'2".