Josef Keiler

Mapping The Lives (which gives Josef's date and place of birth) shows that Alfred, Dorothea and Josef were the only KEILERs at this address on 17 May 1939:
7.05.1939
Kantstr. 20
Breslau / Stkrs. Breslau
Schlesien
/ Deutsches Reich

There are no deportation details shown against Josef.

My assumption is that Josef is their son.

There is a Joseph Manfred Keiler from Germany, who is naturalised British in 1947 (London Gazette); an electrical engineering student. My assumption is this is him. Likely he came to England on kindertransport. Person of exact same name died 2012 in Leicestershire. A Joseph M Keiler married in 1957 in Bristol to Margaret Merrington; a daughter was born in 1958, in Leicestershire.