am I allowed to go to his court date to advocate for him to be given max sentence?
Let's start with a couple basics. First, in any given criminal case, there may be several court dates. You showing up for this purpose at any point prior to the sentencing would be pointless. Depending on the court rules in your unidentified state and locality and the crime charged, sentencing may happen immediately after the reading of the verdict, or it may happen at a separate hearing. It's also possible that he'll negotiate a plea bargain. Again, depending on the laws of your unidentified state, the prosecutor may or may not be obligated to give you notice regarding a plea bargain.
Second, a PPO will constrain him. It won't constrain you (unless it's a mutual PPO). Thus, you're free to attend any public court hearing just like any other person.
As others have noted, however, simply showing up and expecting to stand up and yell, "give him the max!" would be foolish. You should coordinate with the prosecutor in advance.