Here are a couple of things.
1) When he is having a tantrum call 911 and ask for him to be transported to the ER for a psychiatric evaluation for admission. With your consent he can be transported. Just because he calms down doesn't mean he wont be admitted.
2) You mention the cost of long term admission to a psychiatric hospital and that he is adopted. Check with the adoption agency. See if he qualifies for medicaid or any other services/supports.
3) Lock up the dangerous objects in your home until you can get his behavior under control. This does mean locking up the knives you use. Then just getting them out when needed, wash them and lock them up again. You can lock them up in a small tool box from the hardware store.
4) If he isn't getting therapy in addition to medication management, he needs to be in therapy asap.