It is basically undeniable by now because of how in the origin episode the picture he was holding was Mr. Agreste's wife but also because if anyone didn't notice this Hawkmoth and Mr.Agreste both wear a silver ring on their middle finger (Noticed that in origin too). Plus there are the obvious hints like how there's a butterfly on Adriens shoe...or should I say A MOTH!!! (DUH DUH DDUUUHH) An then there is the very itty bitty small mistake of NOT putting the name of gabriel agreste voice actor ANYWHERE.
End credits-NOPE Online cast list-NOPE
I didn't want it to be true but I don't see how it isn't. And to top it all off he basically now knows his ONLY SON is the Boy he has to defeat by any means with the consequences ending in possible life changing injury and or death (I mean come on Adrien is a Model, his dad should at least have the courtesy to tell the bad guy to mind the face)..... AND HE STILL TRYS TO DEFEAT HIM ANYWAY!!!!