Well, the 次 movie that Walden Media will make is "The Magician's Nephew", so since Eustace Scrubb is not in that one, Will Poulter won't either. In "The Silver Chair" (the 次 movie chronologically), however, I would say there might be a pretty good chance that Will Poulter will be in it. I know Walden Media (like a lot of movie producers) like to try to keep the same actors for the returning characters.
I want Will Poulter to returns as Eustace Scrubb in the 次 upcoming Narnia film, the silver chair. Just because he is older doesn't mean he can't. Look at Ben Barnes, he was 26 when he was chosen to play Prince Caspian. Surely there's a way to fit him in to reprise his role.