He was probably starved in the uterus. Nowadays, with scans etc. mums are delivered early if the fetus is not growing (what is called intra uterine growth retardation) and the baby is then taken to Special Care Baby Unit until well enough to go home. That wouldn't have happened in 1914 and the baby would have either been born prem and very small, or full term and very small.
Don't blame your grandparents, it can happen even when the mother is well fed etc. it is just that the placenta wasn't working properly for some reason.
Lizzie