I’m sorry your relationship has ended. I hope you can take some time to heal.
I want to address some of the posters on this thread who seem dismissive, bemused or incredulous at the friendship between the OP and her ex-partner’s son.
My father dated a woman for 5 years after my mother died. She never tried to mother us, they never lived together, and she lived a full & busy life apart from my dad. But we got on really well, and I liked and admired her a lot.
When they broke up, my father said (cruelly) that she had blamed my sister and me for the split (I don’t know if this is really what she said to him).
So, analogously, I was in a friendly relationship with this woman, as the OP is with the son, and at their break-up we, the children (although I was 17 at the time), were unfairly caught up in the disintegration of the relationship.
I am still, nearly 30 years later, good friends with this woman. I see her as often as distance permits, and stay with her when possible. We enjoy exhibitions and walks together. I am much, much, closer to her than I am to the unpleasant woman my father ended up marrying.
So, it is entirely possible for a teenager to bond strongly with a nice adult, and to remain close to them, despite the original grounds of the friendship disappearing.
OP: if you are close to the son, don’t despair. Give it time for the fallout to settle, then simply be available as the friend you were before.