It is hard to be sure but if you feel he may harm himself in the immediate future, you can call 911. They can assess if he is in immediate danger or have him taken in to be evaluated for 72-hours.
You should always take the thought seriously, esp if he has risk factors (depression, substance abuse, family history) but even professionals have a hard time accurately assessing.
Is he getting any other support like therapy? You can tell his therapist or counselor as well.
If you are unsure in the moment, you can call a local suicide hotline and explain to them his behaviors so they can give you guidance.