Hey there @Lillian4 thanks so much for posting, I am really sorry to hear about your son's memory loss. My guess would be, dissociative states could certainly impact memory, but a helpline would be better placed to have a chat about this. The SANE Help Centre are open 10-10pm weekdays, they have a lot of insight into BPD and are good for general strategies and advice.
Do you find this is the biggest challenge in supporting him? That he has differing memories of childhood, or even recent events? What does that conversation look like? Feel free to tell us a bit more and the community can definitely provide some insight