There might be a little more information in an 1986 article published by the alumni magazine of ENS: google would only reveal that he was born in the small village of La Clayette and died of cancer aged 62.
Concerning the wartime hiatus: in february 1943 it was announced by french authorities that people born in either 1920, 1921, or 1922 were compelled to go and work in Germany (STO: Service de Travail Obligatoire). Since Néron was born in 1922, he either went to Germany or fled (some ENS students have described how they fled).
One thing for certain: Néron was "promotion 1943" at ENS, which means he entered the school then, not that he graduated in 1943. In fact Fields medalist René Thom also was from promotion 1943, and also completed his doctorate in 1951, so in that respect the dates are not unusual. The big difference is that Thom was born in 1923, and thus not subjected to STO.