Yes, he will get credit for the time he is doing unless, he has more than one case and it is somehow given to that case. There are a great deal of intricacies in sentencing, so I would need more info to give you a definite answer.
Sure. But you should be aware that in juvenile proceedings there can be other options besides being in custody including home supervision. His family should consult with an attorney who has handled juvenile matters immediately so he or she can appear on Monday to argue for his release. Good luck.
In California, unless an incarcerated person agrees to "waive credits" as part of plea agreement, he or she gets credit for any portion of any day spent in custody. For example, a person booked at 11:59PM gets credit for the full day, just the same as someone booked at 12:01AM, nearly 24 hours earlier.
If your boyfriend's court hearing on Monday is the first hearing in the case (called a Detention Hearing in Juvenile Court), it is possible that the court will release him on home supervision or an ankle monitor if it determines that public safety does not require his confinement, his family is able to properly supervise him, and he will obey the court's rules while released.
Generally, yes. The time spent in custody awaiting the final outcome of the case (sentencing) is credited to the time imposed. The main reason it would not is if he was sentenced on another matter, and was currently serving that sentence.