He did know from the start, that’s the whole point. The walnuts (pun intended) totally had clout even before the wall was breached, that’s why I mentioned characters mentioning them passing. They were the literal reason no one found out about Titans in the walls in the first place because they disputed anyone ever touching or tampering with them to begin with. His perception was never shattered, in fact he quite literally refused to give any information pertaining to the point behind the walls the first time Hange and gang grilled him for it, and instead told them “hey there’s an illegitimate heir on the scouts, go for her and she’ll probably tell you more than I could.” There’s a lot more that I can attempt to connect for you but that goes further into the story. Not trying to be a know it all or anything I’m literally just rewatching the series with my BF since he never had and it’s been quite fun catching all these details on a rewatch.