How about I tell my boyfriend to grow up?
What did your boyfriend do to make you think he was immature?
He told me that I was not supportive of him and his dream of becoming a pianist.
It's important to be supportive of your partner and their dreams. If your boyfriend wants to be a pianist, then I think it would be best to try to help him accomplish his dream!
Why would I do that? If he wants my support then he needs to prove how much he deserves it.
No one has to prove how much they want a dream in order for a loved one to be supportive. You should just be supportive because you love him, and if he fails then you should be supportive of him trying. How would you feel if people kept telling you not to do something that you really want to do?