Hi and welcome
I'm so sorry to hear about what you've been experiencing and I feel for you. It must be very distressing to feel your son distancing himself from you. It is possible that gf is isolating him to some degree if she has BPD/traits, as she will suffer from extreme fear of abandonment and be wary of other relationships in his life so I'd encourage you to be open to him any time he reaches out, no matter what conflict may have gone before. Unfortunately he may also be putting distance between himself and other important people in his life, so there's every chance he will need support in time yet may feel afraid that he has caused too much hurt. It could help if you are able to reassure him that you will be there for him without judgement no matter what and remind him that you love him unconditionally, even if it means that stepping back a little and giving them space is what is necessary right now.
I am sure that you will get sound advice here on how to handle this from others who can further relate and that your T will be helpful too. It's great to see that you are seeking help with this and building a support network, as this is important, along with developing your knowledge on the behaviours that can be expected and how best to manage these. The articles on this site are reliable and I'd encourage you to take a good look around, as well as reading others' posts.
This will help you to see you are far from alone and also help you to know what your son may be dealing with. Each r/s is different and BPD is a spectrum disorder, however there are some common themes with regards behaviours to varying degrees.
There are excellent relationship skills available in the
Tools section on the top header menu, which I'd recommend you read up on and begin to practise. These may feel awkward and unnatural at first, and it takes time to get better at them however well worth the effort to reduce tension. Try not to blame yourself for what has gone before. Experiencing what you have would confuse anyone, and it's clear you wish to make things better. I hope that you find the r/s skills useful and begin to see improvements. Keep posting here, as it helps to share with others who understand what you are going through.
Love and light x