On a scale of 1-10, it's a 15. Plan, access to a firearm, issue's why he's had a plan, weren't addressed, you didn't mention he was placed in a mental health unit for care. This is a serious, emergent situation, if after reading this, you still haven't been able to raise him, call the police ASAP so they can check on him. He needs to go to the hospital. I am speaking from experience. It's starts with thoughts, it leads to shows of suicide attempts, cries for help, it leads to plans, and finally, resolution. Even, if you get him on the phone and he appears okay, cheerful, calm etc...no. He needs to go. Because if he calms across, calm, suddenly, this is the eye of the storm. Please, get him help.
ETA: Please, I can not stress this enough. If you are unable to raise him on the phone, do not, do not, yourself go to look for him. If he's in the throws of a crisis, in that moment, he's not thinking clearly, and it's unsafe. If his norm, is to respond to your texts...something is wrong. Never, second guess your intuition. Please keep us posted.