Trigger warning: suicide, self-harm, eating disorders, PTSD

I desperately need some advice. My boyfriend has recently been talking a lot about not wanting to be here anymore, and just told me tonight that he has a plan and a date for it to happen - but he refuses to tell me the date and plan. He suffers with depression, anxiety, PTSD and an eating disorder, and has previously self-harmed and attempted suicide once or twice in the past.

I also suffer with my mental health and have had suicidal thoughts before, so this is not wholly unfamiliar to me. I have tried to listen as much as I can and suggest things but he really does seem to have made up his mind. He says he wants to say his goodbyes and has asked me to look after his dog for him.

I'm in two minds whether to blow the whistle and tell his mum and/or healthcare professionals about this. I obviously would do anything to prevent him from taking his life, however I also don't want to break his trust in me. What if he doesn't go ahead with the plan for now but plans something else and can't trust me to tell me that he's going to do something then?

Please help. He didn't give me a timescale for his plan but he said 'soon'. Thanks in advance.