I’m sorry to be the bearer of bad news, but you’ve asked for an honest opinion so here it is…
If he refers to her as his “significant other” then, yes, you are the other woman and frankly he has told you as much when he says he considers her his “significant other”.
My advice to you would be to stop pursuing this guy and to look for someone who is prepared to make [b]you[/b] their significant other.
More, chances are that if this guy is cheating on his current significant other with you, he’ll cheat on his next significant other too.