I'm not going to allow my boyfriend to get a piercing
What is your reason for not wanting to allow your boyfriend to get a piercing?
Because I don't like those kind of thing, plus he will look like a punk, and he doesn't need that.
You should not tell your partner what to do. Suggest that he reevaluates his decision before getting a piercing.
I'm doing this because I love him and will make him a better man.
It is not a good idea to try and control what your partner can and can not do. If your partner feels like they do not have their own personal freedom, odds are they will find someone who allows them to in a relationship.