What do I do if my boyfriend came to the US when he was under age and my ex-boyfriend is threatening to kill him or deport him?

First you current boyfriend should see an attorney to check out DACA possibility; second, if the threats of killing are serious or you regard them as serious, you should go to the police.

Your boyfriend needs to call the local police and report the threats on his life. The local police in many jurisdictions will take action against your ex-boyfriend. If your ex-boyfriend it's arrested, then your current boyfriend may be asked to be a witness. He should cooperate. As a cooperating witness, he may be protected by police and even qualify for a visa as a witness if the police agree. That said, he should seek an experienced immigration attorney to work with him due to his fear of police. Perhaps, there are other complications. Usually, local police do not work with immigration on such matters. They work with victims of crime. Your boyfriend seems like a victim if your ex-boyfriend is threatening to kill him and you believe that your ex will do so. You may also need an order of protection, so you need to take appropriate action for yourself. Good luck. The above is general information, not legal advice, and does not create an attorney-client relationship. I strongly recommend an appointment with a competent and experienced immigration attorney as soon as possible.