Currently, the king and the knight share too much
Agreed.
Just some ideas on that.....
Knight (we've labelled him that now!) could be younger, smaller armour around the hips maybe a weapon sheath, sharper shoulders, less slouched pose with chest plate protuding more, different chin and facial structure and lose the moustache as I mentioned.
You could make another grunt character thats hunched over.
King could stay same shape, add distinguishably decorated pauldrons with cloak attached.
Probably lose the similarities between character apparel, switch colours around etc.
Knight's armour could be practical while the king is adorned with more decoration, gold chain etc.