F.W.
Not a clue as to 'why' all of the sudden.
BUT, for years, my kiddos called my fiance "Mr. First name" as a sign of respect that he was an adult.
"Mr. First Name" died in april of 2013. Over xmas, they ALL referred to him that way.
Perhaps this change of name is because you will soon be a relative? (a proposal soon??) Other than that, I have NO clue.
BUT, my kiddos, when younger, never referred to ANY adult (when under 18) by their first name alone.
I am an 'army brat', and a former military spouse. Other than a few life long friends that were 'army aunts/uncles'...all adults in their lives had a 'title'.
Five is young (IMO) for any child to be on a first name basis with an adult...no matter the relationship.
Best!