Do you think it's okay for me to ask my boyfriend not to drink on his night off?
Why would you ask him not to drink on his night off?
He drinks too much and it affects his work performance the next day.
I'm sure you're frustrated with your boyfriend's drinking. It's bad to drink so much that it affects his work performance the next day. Have a discussion with him and see if there's a way to get him to modify his behavior.
I'm planning to leave him. He's becoming boring and he's mean.
You may be right about leaving him. But you should talk to him directly as you first suggested. If his work performance is impacted by his drinking, it would be a good conversation for him to hear.