Where was he evacuated to and when? There's a Reginald Allum in Hastings (born in 1930), and Robert C? Allum in Watford born in 1927.
Have you tried looking for other relatives who might be at the same address? Is he still alive or recently died? May have to wait a while for his details to be unmasked.
I'm still trying to find my grandmother's entry - only can find her parents, even though she would have been of school age.