When was it understood that a male haemophilia sufferer can't pass it on to his sons?
Using Google Scholar, I've found the oldest publication on this item.
1911 - "The pathogenesis of hereditary haemophilia".
In this article you'll find a complete synthesis of all the works done about haemophilia till 1946.

OP adding a summary, in case of future link rot: The linked article is from 1946, based on research in the late 1930s and early 1940s into the statistics of haemophilia. It's attempting to estimate the mutation rates of X chromosomes in males and females, and to test if X-Y crossover allows the haemophilia mutation to skip generations. It assumes the knowledge that leads to the conclusion that a male sufferer can't pass it onto his sons.