The detective has no control what a prosecutor, especially in a different area, does regarding a prosecution. A judge doesn’t either so some judge wanting it all of any possible charges at one time is irrelevant. The da files charges when they believe they are warranted and they have adequate belief they will be upheld in court.
if your son has a valid defense, he will get to present it in court during the trial. What the detective said is not any sort of defense. In fact it’s meaningless. Not being involved and able to prove it is a defense.
as to the statement about only one person entering the building; how would the detective know? Unless he was actually there, he can’t possibly know with any certainty.

