This is not legal advice and this response does not create an attorney client relationship.
The answer depends on how he was placed in the group home. The implication from your question is that the group home placement was ordered by a juvenile court in connection with a delinquency matter. If so, I agree with the other attorneys' answer that a bench warrant will issue for his arrest.
If, however, his placement was ordered by a juvenile court in connection with a dependency proceeding, it is far less likely that an arrest warrant will issue. His age would make it his choice in this scenario.
He can stay in jail until he is 21. My former client who kept running away from group homes ended being charged with special circumstances murder. Not the case here, but it reminds me when people can't follow the rules, they are bound for prison.
The above information does not establish an attorney client relationship nor is it meant to provide legal advice.
If your boyfriend was placed at a group home because his parents were not willing or able to provide him with an adequate degree of care, supervision and/or guidance, then he was placed at the group home for his own protection, through no fault of his own. In these kinds of situations, the state steps in as a surrogate parent, and just as child is no longer obligated to obey the rules of his or her parents once the child becomes an adult (turns 18), a child living at a group home under the "state as surrogate parent" scenario I just described is no longer obligated to obey the rules of the group home. He is free to leave. With that said, he might want to think long and hard before leaving the group home, however, because many states will allow a child to continue to reside at a group home past their 18th birthday (for example, until age 19) once the child says "thanks but no thanks" and takes off, he wont be able to return to the group home should he learn that free room and board is a pretty good thing.
If your boyfriend was placed at the group home as a part of a sentence for a criminal act he committed as a juvenile, however, if he leaves the group home he can be arrested and charged with violating his sentence.
What can happen to your boyfriend depends on why he was placed at the group home. The court will also look at why he ran away and whether he has run away before. If your boyfriend is caught he can be placed in jail rather than juvenile hall since he is 18 while the court decides what to do with him. He should contact his attorney to get advice about his specific situation.
Autumn Paine is an attorney licensed to practice law in all California State Courts, and the United States District Court, Northern District of California. The information provided on this site does not create an attorney-client relationship. It is recommended that you contact a lawyer directly to discuss your particular legal situation in detail.