Seems to me
It seems to me that there must have been ample time to discuss your polyamorous relationship with his new overseas friend. "tell me about yourself?" would be an appropriate time to broach the subject. I don't think that your feelings of jealousy are unwarranted. It seems to me he isn't being honest with his new friend and that isn't fair to anyone. If you have been dating two years and he is moving to be with you and move in with you that seems like a big part of his life he is not mentioning. Only advice I can give is talk to him and let him know it bothered you.