How to get SO’s parents to like you??

I know it takes time, but coming up is me and my bf’s one year anniversary. His parents still don’t approve. They’re very strict, I’m his first girlfriend, and neither of his two siblings (older + younger) have ever dated.

They don’t mind me as a person, just the idea of me and my boyfriend dating.

Our relationship is getting pretty serious, we’re both on the same page with our future (marriage, etc.) My parents are super supportive, my mom even will sometimes refer to him as her “future son in law!”

What can I do to get them to be more comfortable with me? Every time I go over to his house, I bring some sort of gift and try my best to be as polite as possible. Any suggestions would be really appreciated. Thank you!

By the way, we are in high school, and everyone else in our lives (friends, other family, my therapist) entirely supports our relationship.