Has your boyfriend got any relatives or close friends? They might be able to intervene, plus, they might have some take on what the threatening call was about.
Crisis line would be an option also. You cannot take it on your head to become a suicide watch counsellor. If he says he is going to kill himself you have to take that how its said. You have a duty of care, grim though it may be, to inform family and if not the authorities.
Best of luck.