i've noticed that a lot of the times when the doctor actually uses ian's first name, he's referring to ian and barbara collectively. which makes sense, cause it would be a little weird to use barbara's name but just keep calling ian "dear boy" or "chessington" or whatever. like "susan, go find barbara and... that young man." most of the other times are him speaking either to himself or to barbara/susan/vicki, not directly to ian. which means he could be using ian's first name all the time if he wanted to, but he chooses to only call him increasingly incorrect variations of his surname to his face just to mess with him











