Thanks for the addition JenB. I don't suppose the enumerator would have checked if 'ex-soldier' was correct, but yes, he may have been discharged [or not enlisted] as unfit to serve.
ADDED: His mother had died in 1914 when he was 11, 2 younger brothers and 2 younger sisters were in industrial schools in 1921, another younger brother I have not yet accurately located in 1921. Probably the army seemed a good idea.