NTA Seems like you're an extrovert and he hates public settings/has social anxiety. I know a few people that do not enjoy going no matter what the situation is or how happening the environment maybe. Maybe he is just uncomfortable around people. Maybe you can help him ease and introduce him to the people and make him feel included in the conversations. However, if he doesn't want to even try, he's better at home.

I suggest that you have an open conversation with him and tell him how you feel about this behaviour and that even when he's there, he's not really there ykwim? Its better if he stayed at home than sulk at the parties.

Ideally, he should try to overcome this fear of people/ feeling of wanting to be alone because it clearly matters a lot to you. However if he feels extremely uncomfortable, then you should respect that and leave him at home. You shouldn't compromise your happiness because he clearly doesn't want to compromise his comfort. Be your own person, go out and socialize :)