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I don't think he would be investing this time and energy into you and building a relationship with you if he still desired the ex. Yes, he texted her Happy Birthday...maybe more from being a good guy and not from any real desire to be with her. I would try not to get too hung up on this and just enjoy your time with him and let things play out as they will...no need to force things or heavy conversations, just enjoy what you are building and it sounds like he is too.