Hi Sian,
I'm very sorry your partner and you also experienced such a tragic loss. Losing a family-member like this is horrible. How are you coping yourself?
Was suicidal ideation a known issue of your partner's son? Was this something he had been struggling with?
You are concerned about your partner. People with BPD often have a deep fear of abandonment, the fear is often irrational yet when real losses like this happen, this can have a very deep impact. The irrational fear all of a sudden becomes very real.
We have a resource here that specifically deals with suicide loss. This loss is very recent, but at some point you might find this resource helpful:
Self-help Handbook Recovering From Suicide LossTake care and wishing you strength in this difficult time