Frankly, I don’t see the problem here. You’ve only been dating 8 months. He told you he was spending thanksgiving with his dad and his children. He didn’t drop this news on you. You had plenty of time to make alternate plans, so that’s on you.
If you were disappointed he didn’t ask you to join, you should have told him and listened to his reasoning and then maybe compromised. Like you two do your own thanksgiving celebration the week before or after. Instead you somehow tried to test him or something? You’re 51. Use your words. Communicate.