Honestly, if you think he's planning, rather than "just" would rather be dead, I'd call the police and ask for a welfare check.
He's put a horrible burden on you, in all honesty, and I'm sorry you're going through this. Having the police check might give him a wake-up call and/or get him the help he needs, if he won't call the CMHT/Samaritans/A and E
I am giving this advice as someone who has previously made several serious but unsuccessful suicide attempts. One was dumb luck it was unsuccessful; two were unsuccessful because of intervention by strangers. In each case, I was committed to dying, and hadn't discussed this with anyone - so based on my very unrepresentative sample of 1, I wonder if he's told you because, at some subliminal level, he wants you to get him help - even if he'll subsequently be angry/irritated with you for doing so.
He's obviously not in a place where he can feel compassion for others, but it's putting a hell of a burden on anyone, especially someone who is not particularly close, to give them this info. In your situation, given that he's refusing to seek help (or is unable to - more to the point) I would personally contact the police and explain that you are concerned for his well-being, that he seems suicidal but unable to access help for himself, and ask them to welfare check him.
I'm saying police and not ambulance as it's easier for him to get rid of an ambulance, and the police can call one if they feel it to be necessary.
I would also - gently, and once this crisis is past - explain to him that, whilst you feel for him, you will have to disengage, whether from him completely, or from these kind of conversations, as you are not able to take on responsibility for his welfare over this, and it's causing you other issues as a result. It can be incredibly emotionally draining dealing with someone who is suicidal, in these kind of situations where you are being pushed to take a role that isn't appropriate - and you can frame it as not having the skills/if you do have the skills, ethical issues about having a therapeutic relationship with a colleague etc. I know other posters might say I'm being harsh or unsympathetic (which is why I added my personal experience above) but the bottom line is that depression is a selfish illness that leaves you with no energy to look outside your self in any meaningful way. And although the effects are different, that makes it pretty ghastly for someone else to support you, as frankly, you don't even recognise the pain and effort it involves when you are as ill as he sounds right now. And you need to look after yourself in this process too - particularly in these circumstances. Sorry you are dealing with this 