According to a fan@2ch who attended VS Arashi’s recording on Tuesday, whenever the cameras were rolling, Sho-chan called Aiba-chan “Aiba-kun”, but when the cameras were not rolling, Sho-chan called Aiba-chan “Masaki”.
=> So I assume that in private, Sho-chan always calls Aiba-chan “Masaki”.
(because I can assume things like whatever I want to!)