Unless there is a bigger backstory here, then it sounds like a massive case of miscommunication and assumptions on both sides.
If you want the dynamics to improve then at some point someone has to take a bigger step and there is no reason why that cannot be you.
Generally speaking, people respond well to things like vulnerability, honesty, and being asked their advice/expertise. Being open/vulnerable is disarming and opens up conversation, while being asked advice/expertise is flattering to most people.
You say you are doing some decorating at the moment - this could be a really good opportunity. I would contact your son's GF - say you are in a real dilemma over something decorating related - eg. what colour scheme to pick, curtains, a piece of furniture, carpet, soft furnishings, a light fitting... whatever - and you would REALLY appreciate her input on it as she's got a really good eye for these things. Invite her to lunch or tea or whatever to discuss ideas. Just her. Discuss the advice you want... and also, use it as an opportunity to open up.
I notice you feel that you come across stand-offish etc; people generally respect when someone has self-awareness. So you could say something like - "I'm so grateful for your advice on this... and it's been so nice to see you today. I know sometimes I may come across as... (insert whatever you want here)... but please know it is not my intention and it comes from a place of my feeling xxxx (again explain the context)". Explain that you really like and respect her and would love to get to know her better. Don't be afraid to share a story about yourself or whatever - doesn't have to be too heavy - can even be funny or about an embarrassing or silly moment in your life - which to many people is a scary thing to do, but doing that is disarming to another person and can open up a really nice dialogue. You may well find you have things in common you didn't know about.
If she reciprocates with her own stories and/or seeking YOUR advice about something, welcome that and offer your feedback/suggestions in a positive non-judgmental way - keep it friendly and fun.
If the above goes well, then acknowledge that you have really enjoyed it and say you would really like to invite her and your DS to lunch or dinner... or perhaps go together to a show or something where there are definite dates involved. If she says she needs to check her diary, that is okay - most people do - but don't just assume it's a dismissal, and take into account that people are often forgetful or get caught up in things and honestly forget to follow up with stuff. Give it a couple of days and then call - say again how much you enjoyed the day you had - and follow up re arrangements for you all to get together.
If you've asked for her advice on decorating - and she has suggested something you like (wallpaper or whatever) - you could even use that as an opportunity to get together. Invite both of them to go shopping with you to pick out whatever it is and suggest lunch afterwards or something.
I know it sounds like it's all work on your side, but it's possible that they're thinking you're not bothered or don't need their advice/help. I'm a very independent strong person but that can be a double edged sword at times.
I'm also the first person to say "don't chase people" - but in this scenario, I don't see it as chasing, more a case of taking steps to break the pattern and create a new, better dynamic. If you open up a new dynamic with the GF then it may also do so with your DS.
If you try the above and/or other steps to shift the dynamic, are making efforts to communicate etc... and you get nowhere, then that is when I would say "don't chase" - find amazing things to do in your time, enjoy interests etc... By all means remain friendly and keep dialogues open, but if you are super busy in your own life, it is empowering and it will feel less overwhelming in terms of when, why or how arrangements are being made (if that makes sense?).
Sorry - I know this is long but I hope it helps.