How do I tell my boyfriend's parents that their son is depressed and possibly suicidal?

Suicidal? It may difficult to tell his parents, try. I suggest you contact a suicide prevention phone line-supoort group asap.If his parents are not listening, and the situation is immediate, call the police and they will send help.I am sure that you have tried to discuss his sadness with him;you must do the right thing.He needs to get help and you need to disclose this emotional situation for his sake+you.
Any way you can. This is information that needs to be passed on to the parents. Imagine how bad you would feel if you did not because you were afraid of the boyfriend's reaction, and then he hurt himself !