"any media he has posted older than 24 days only shows as a blurred gradient in the app."
Ahhh... okay... possibly a different issue them? It sounds like the post is there, but the media file is not displayed.
That might be more to do with your server's media storage provider not displaying it properly, or perhaps your server has some resource-saving settings that affect this.
AFAIK there's nothing in Mastodon to stop a post's photo being visible.