I'm not quite sure how this is going to work out.
Apparently, now that he's in the adult section, he can apply for bail and may be released pending his new trial. (Bail doesn't seem to be permitted in the juvenile system.) Whether bail would be granted to somebody who was arrested for fleeing the country is another matter but, hey, this is the affluenza kid. He's already got away with murder.
Being admitted to the adult prison is likely to open another can of worms for him though. If the prosecutors succeed in re-opening his case he would then probably be re-tried as an adult and would almost certainly be given 10 years for each death plus whatever the judge seemed appropriate for skipping the country. All that would total more than 40 years.