You are in a difficult predicament. The person who you are trying to help is refusing all of your efforts. That further complicates the situation.
I believe your concerns are valid. Your friend may be experiencing psychosis. Most people with psychosis are not violent but they can be, under certain circumstances, such as in the scenario you have described. Though it is unlikely, that scenario is not outside the realm of possibility.
There’s also the issue of possible suicide. He stated that he knows that he will not be living by the age of 19. That statement might be indicative of a delusion but it could also mean that he is planning to end his life.
Psychosis jumbles the mind. It is a struggle to know what is real and what is not real. Psychosis can be very frightening but it is treatable with medication.
My recommendation is to contact the local mental health crisis team and ask for their assistance. They can come to his home and evaluate his condition and determine if hospitalization is necessary. You can identify if mental health crises services are available in his community by doing a Google search or by contacting the local hospital or health department.
In Florida, an individual is eligible for inpatient hospitalization (even in the absence of health insurance) when they meet at least one of the following criteria:
“An individual is unable or refuses to make responsible decisions with respect to voluntary placement for treatment AND either (1) without treatment, incapable of surviving alone or with the help of willing family or friends, and likely to suffer from neglect or refuse to care for himself/herself that will pose a real and present threat of substantial harm to well-being OR (2) danger to self/others, as evidenced by recent behavior.”18-Year-Old Friend Has Paranoid Delusions & Refuses Help