Mixing age regressing and fledgling Jon would be fun I think.
Jon getting to be little and Elias hand feeding him cooing over what a good boy he is when he manages to latch on properly. Jon whining when he’s hungry because when he’s little he doesn’t have any of the guilt, he just knows his stomach hurts and his maker can fix that.
Jon being especially greedy when he’s little, until he drinks far too much and then needs to be cuddled because he’s still in pain, only now he’s too full. Elias cleaning his face off telling him what a good boy he is.
Just Jon being a little feral when he’s small, because it’s all instinct then.