My great grandfather was born in 1891 and we are curious as to why he didn't serve in WW1. Is there any way of finding out why he may have not served?

We are pretty certain he didn't serve as he fathered a child during WW1 (with a lady whose husband was at war) and he raised the child.

If anyone can help his name was Arthur Newton, born 1891 in Battersea London. His child was born in June 1918 so he had to have been around in 1917.

If anyone can point me in the right direction or offer any help it would be greatly appeciated.