For clarification, when you say "dating" do you mean you didn't consider yourselves "in a relationship" even though it's been 5 years?
From my point of view, it sounds like you've been experiencing poly the entire time he was dating both of you.
What about moving in together changes that?
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker