Underweight Teen - almost 20 - Male

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I'm not exactly buff myself at 5ft 6 and I hover around 8 stone to 8-4. My poor son at 6ft seems to have gained my build. It's not a great look for a soon to be 20 year old. He does eat, though it must be said, since he has moved back to the UK, I no longer have an eye on him. I don't think he has an eating disorder such as anorexia but I have asked him to go to the doctors. He was always thin throughout his teens. He probably weighs not much more than me. The guys in my family are all tall, taller than him and built to their size. We are the unusual ones. I was partly expecting him to start filling out a bit. I'd be amazed if he was even 9 stone. He's too skinny. Has anyone else experienced a skinny one? Advice?

No advice per say but my husband is 6 foot odd and looks like he'd snap in a strong breeze, some people are just built that way. He may also feel very self conscious about it, my DH does and won't wear shorts or go swimming.

My DP looked like a piece of string with knots in when I met him about 20 years ago, he's nearly 50 now and has finally started to put some weight on.
This may be because he eats DS's leftovers

Sorry that's not much help

My 16 year old son is the same, and he eats loads. He's going to start some weight training to bulk up.