If he didn’t want you to go, he wouldn’t have invited you! YOU were the one who made things awkward by suggesting it might be awkward if you went. How would you feel if you invited someone to do something and that person was all, “Well… I don’t know… it could be awkward…” Yeah, NOW it might be. Tell your boyfriend you were being irrationally insecure and, of course, you would like to go to the Halloween party because you are part of this family (I mean his son lives with you!), and you are grateful to be included in these family activities. Then, use this as an opportunity to have a heart-to-heart with your boyfriend about your role in his son’s life, both now and in the future. And consider whether getting married might make “defining” your status to outsiders, as well as to your boyfriend’s son, a little easier, and if that might make it worth it to you guys to take that step.
If you have a lifestyle that allows it, you could winter in the warm climate (either together or on your own) and spend the rest of the year in your girlfriend’s home town. Or, you could have a long-distance relationship until one or both of you retire and/or can have more lifestyle flexibility where you can winter in one place while retaining a home base elsewhere. Or, you could change your relationship to a friendship, thereby keeping her in your life but freeing each other to pursue the lives that bring you the most joy (you in the warm climate, and she close to her family). If none of those options work, you really have to decide what is most important: warmth in the winter or being with the one you love.
If you have a relationship/dating question I can help answer, you can send me your letters at firstname.lastname@example.org.