I think my boyfriend wants to be a rockstar
Why do you think he wants to be a rock star?
Because he plays guitar all day. Unfortunately, he completely sucks.
It's fine to have hobbies. When your partner has a dream, it's expected that you support him. It will make your relationship stronger.
His dream is stupid and he sucks so he will never reach it.
If it is something that he enjoys, you should support him. He probably is just having fun so you shouldn't be angry with him.