Board Thread:Manga/@comment-29919227-20160909220906/@comment-26932251-20160909225505

I think the safe bet might be that Zeke is Grisha's son for one practical reason, Zeke mumbles to himself about his father, and that means that Isayama would need to, if Grisha isn't the person in question, introduce yet another character and probably make him pretty compelling, probably would have to devote at least a full chapter to just this father character in order to explain how this character set Zeke and presumably Grisha on their way.

Easier to just make Grisha the father and also Zeke could pass for 30 and Grisha could pass for someone in his 50s, it's totally possible.