Don't take it personally. It is not about you. He is in his early 20's and looking to experience and create life on his own terms. I remember how I was when I was in my 20's...I loved my family dearly, but hanging out with them was the last thing I wanted to do. I would go 3-4 months without talking to my parents.
I think the more you try to force the closeness - the more he will pull away.
It sounds like you have placed blame on his "antisocial girlfriend". Try to work through those feelings and release them. They will both pick up on that underlying negative energy which will only increase the tension and distance. He is doing this of his own volition, and it has nothing to do with you. It has everything to do with him finding his path into adulthood. This is a stepping stone.
I know it hurts. You are grieving the loss of the closeness that you shared. The good news is the closeness will come back. He knows how much you love him. This is a stage in his life; it is transitional, temporary - not the beginning of the end.
Try to shift your emotions and your energy away from focusing on what you don't want or don't have. Focus on how full your heart is with love and gratitude for your son and family. Try to set aside 15 - 20 minutes in the morning and the evening to do this.
Keep a list of all the positive traits that you appreciate about his girlfriend. Focus on acceptance and gratitude because your son has found a partner from which he will learn valuable life lessons.
Journal - if you are feeling negativity and you just need a way to get it out - write it all down on paper - purge. Don't censor yourself - no matter how ugly your words and feelings might be - get that poison out of your mind and body. Then tear it up or burn it. Once that's out - then fill your mind with unconditional love, acceptance, support, patience.
Best wishes
M.