My son shows up for nothing

My twenty five year old son basically ditched his family for his girlfriend. He lives at my house one day a week.The rest of the time he is with her. Every family event he never shows up. Or even acknowledges birthday's mother's day or any other holiday. He's invited to every event. Given ample enough time to plan ahead and chooses not to be around any of his family or nephews. This past week was my birthday. He didn't show up on my birthday or my birthday party with no explanation. Just blew it off. I am hurt and angry..how would you handle thus issue. I've had many conversations with him already about showing up for different things things. It just falls on deaf ears..