janc I do sympathise. I have faced very similar problems - my DS1 has just been discharged from a MH ward. We could only watch as he deteriorated over a period of many weeks without any real support - went to A&E 3 times and fobbed off twice with diazepam to calm him down before real action was taken.
As your son is phoning you every night he clearly needs more support than he is getting. If you think his threats of suicide have real foundation and that he is not safe, he needs to be taken to A&E - not left to go alone - and may need admitting to a ward for a time. Beds on acute wards are hard to come by, but if he severely in need, one will be found - refuse to be fobbed off!
My DS is, thankfully , in a much better place and will now have community support at a day centre for another 3 months, It has been a hard journey for him - and us (one of several), but we o what we can.
I hope you can help your son to find the support he needs. The systems are so inadequate we do have to shout hard and long at times. xx