I remember when my brother went to serve his two years National Service they sent him on a special course to ‘build him up’. He was 5’9” and less than 9 stone. Little did they know that, although you could play a tune on his ribs, he had an enormous appetite. After four weeks in Blackpool, being fed large meals and taking part in body-building exercises, he was still as skinny as ever. When he was de-mobbed two years later he still hadn’t put on any weight.
One of my sons is the same. He was so light, as a baby, that the doctor was worried about him, but he was as fit as a fiddle, and was walking at eight months!