What's wrong with telling him exactly what you've told us here?

@TymoteuszPaul: Can you clarify what you mean by that? I cannot imagine listenig the boss's s"sins" (like it done here where the boss could just say "that's a conicidence" would go well..

@guest Why not? Though those are not sins, just points of disagreement between OP and the boss, how OP sees them, and hopes to resolve the issues.

@TymoteuszPaul: I know people like that (a teacher and a uni prof). Saying "you refused my holiday because you don't like my hobby" will not make the problem go away. The boss will say "No, I refused your vacation because of this reasons" and will think worse of you.

..at least, in my experience. Of course, the boss can be a great human. But one should think about this possiblity before "simply telling him that".

What exactly is the boss? Manager? Company owner?

It feels like there is something missing here. Everyone does something out of work. Why would being in a band cause these kinds of problems? It's baffling. Might something else be going on? Of the four things listed that the boss is doing now, only one is related to the singing, and that might just be him trying to be funny or failing to make a human connection. I fear we won't be able to answer this question without knowing what the boss is really thinking and why.

@BittermanAndy I disagree. A big part of Workplace.SE questions are related to someone else's behaviour, which is creating issues and is unexplainable for the OP. A good answer would suggest a way to uncover the root cause or handling the issue independent of it. Requiring a user to do all that beforehand would make Workplace.SE obsolete.

@Chris fair enough, it just seemed to me so unlikely or weird that these two factors (the hobby and the change in treatment) could be related, that answers would be in jeopardy of not being able to address the actual problem. But I suppose that's just what might happen. I will read other people's answers with interest.

Are any of your band’s songs objectionable?

@BittermanAndy it might be as simple as "boss is a twat who has a problem with metal music". Which isn't that unreasonable, as it seems to be fairly common.

You are not punk or metalhead if you are thinking this way. Write a dedicated song.

Some bosses disrespect any hobbies. Some bosses disrespect non-technical hobbies. Some bosses disrespect hobbies that are not up to their lofty standards. Some bosses disrespect hobbies that make them feel that they are beneath you. Some bosses envy hobbies which allow their workers to shine. One reason not to reveal one's private life at work. However, it's done. Do your work well, but don't overstretch. Prepare your way out, but perhaps they'll get used to the idea in due time. And if they comment on your singing then tell them that in terms of stamina it compares to competitive sports.

I'd leave the company asap

@BittermanAndy A possible scenario is that the boss's wife left him for a metal/punk band member and he was too weak to confront the other guy (or smart, ha) but now he has the opportunity to take it out on his employee. The fact the boss is running away from even talking to this guy shows his character too.

Wrt. your holiday, whats your jurisdiction? In some jurisdictions, vacation time can not be refused without very good reasons.

@Polygnome I work in France, and you can refuse holiday with a reason, I'll ask him why.

If the boss has looked you up on social media in your role as a singer, he may have religious or moral objections to your conduct. For example, if an employee was, say, a webcam stripper in their spare time, the boss might think this would bring the company into disrepute. Different people have different standards as to what is acceptable.

How did he find out? Did it come up in conversation, or were you unable to commit to working late because you had a rehearsal? If its the latter then it might be less about your hobby and more about him having unreasonable expectations of your commitment.

Is your role customer facing?

Some people are surprised. But there are many countries where punk and metal music can get you in trouble. In Russia, it can even be trouble with the police.

@Justas Why would that even matter?

@Num Lock customer facing role represents and gives impression about a company. Salesmen may wear suits while devs can be with crocs.

@Phil I don't know how did he find out, most likely from a youtube video or on social media. We usually have rehearsal on weekend.

Its daunting but this is a really good time to look for other work, well actually its a horrible time with COVID if you're in a lockdown scenario... but you sound like you have confidence in your skills and experience, its time for an upgrade. Its really not worth the stress to keep your current post, you deserve to be comfortable in your place of work, you're not a rookie any more, shouldn't have to be treated like it.