Looking at DS - birth 3.75kg (75th centile), 12m 8.5kg, 14m 9kg (9th centile). Currently 14kg, has been since he was just over 3 (he's now 4.5yrs). The puts him on the 2nd centile: so still perfectly normal. There are children out there lighter than him still.

He eats like a horse and has the same metabolism as his parents had. What a shocker! If anyone were to refer him for failure to thrive at 13m I would laugh and point out he eats, he develops, he is as skinny as expected given genetics.