Supervisor's feelings toward publishing prior work without him
Any work you did at a prior institution—particularly something you did before starting a PhD in a different field—before joining your present institution is not really any of your current advisor's concern, unless working on it involved a significant distraction from your assigned work. (That is, he was paying you to work on project A, and you spent all your time working for project B.) It doesn't sound like this is the case, so I wouldn't lose any sleep over this.