I've just browsed the 1931 birth registers for AMJ and JAS quarters and there's no Trevassa (or variation) that I can see. If he was born 28th May 1931 then I think he would have been registered in May, June or July.
Far more likely to me is that the death entry on the index has been copied incorrectly and instead of his birth date being 28th May 1931, it should say 28th May 1939, which would fit with the Morris WL Trevassa registered in the 1939 JAS quarter.