He called him Jou-chan in the first ep and switched to Jou-san in the second episode
I think he viewed him as a beloved uncle or father figure so I remember him saying Jou-chan. Later on after all the revelations, he must have felt some distance from Jou so he switched to Jou-san.
I watched Taishi in Minami-kun and Blood and Bones. He's a funny actor tbh