At your son's trial, the "burglar" and his sister will likely testify that your son was involved, and the other boy will testify that your son was not involved (or at least that's what he has told you). It will be up to the judge to determine who is more credible. However, if the other boy testifies that he was not involved, it will be difficult for him to also know whether your son was or was not involved unless he was with your son elsewhere at the time of the burglary.
It would be interesting to see what type of physical evidence, if any, that links your son to the crime. Some burglary charges have fingerprint evidence at the point of entry, or on the TV, etc etc. Without any physical evidence, only the testimony of an accomplice, it may be tough to convict. Remember that possession is 9/10th of the law, meaning that the fact that your son had ZERO items in his possession should carry much weight (he helped with a burglary but never split the goods? no payment whatsoever for all that hard work?). Hopefully, the boy that pled to the criminal trespass will admit that your son had nothing to do with it. Also, and most importantly, I hope you've got a great attorney!
Also, in juvenile, you're allowed to take depositions, so this "new" witness should be deposed to find out what his story is before starting a bench trial with a judge. That will tell you the real "effect" of his acceptance of the plea deal on your son's case. Some exploration of their motives to frame you son as opposed to anyone else in the neighborhood/school would be useful too.
Your son's attorney definitely needs to depose or take a sworn statement of the boy who just took the trespassing plea to see how his testimony will affect your son's case. In many instances, law enforcement doesn't do much in the way of actually investigating these juvenile cases once they have recovered the stolen items and caught the main perpretraitors. So it is important to take statements from all of the witnesses the state has listed.
In many instances, the state is more willing to file a delinquincy petition in cases when the only real evidence against your son may be that he was riding with the other kids on the night the crime was committed, even though he never set foot in the house. Mere presence is not enough, there must be some evidence that he actually took part.
I am assuming that the other boy who took the trespassing charge had admitted to his attorney that he went into the house with the other kids whot took the 3 items - which would make the trespassing a pretty good offer. You definitely want to know what this child will say about your son's involvement before the trial is rescheduled. If he still says your son had nothing to do with it, then your son's attorney might want to list him as an additional witness. If he has now changed his story and implicates your son, that may change his chances at trial.
Make sure your son discusses this with his attorney (I am assuming you have one since the case was set for trial. If he doesn't, you need to get him one.) Good luck.