If, as sounds likely, the query referred to by MaureeninNY is indeed your father, then it seems he does have more knowledge than you appear to know.
It would seem, therefore, that Denson was Russell's stepfathers surname and not the 'adopted' name at all.
His mother is Eunice Christine Cason birth reg'd Mar.qtr.1888 Leicester. By 1911 (not allowed to post details) she appears to be working in Brighton, Sussex.
Her children referred to in the 'query' appear to have been fathered by a Charles Alfred Russell Lepage/Lepage but they were not married. On 6/1/1919 at Christchurch, Brixton, London Eunice Christine Cason 30, spinster, dau. of Henry, married Reginald Denson 33 (of Calgary, Alberta).
I can see, so far, 2 likely births:
Zoe Le Page birth reg'd Sept.qtr.1911 Steyning, Sussex - mother maiden name Cason.
Russell Lepage birth reg'd Sept.1912, Brighton, Sussex - mother maiden name Cason.
So a) Russell was actually a year older than thought being born 1912 not 1913, b) possible sister born the previous year, both born with surname of Lepage/Le Page and the subsequent surname for Russell of Denson was actually from his stepfather and not the family with whom he is said to have been adopted. Unfortunately, no formal adoptions were recorded until 1927.
From a quick search of records, I can find nothing at all for a Charles Alfred Russell LePage, in births, census and can find no WW1 death for him either so who exactly was he?
Annette