The attorney you spoke with is correct in the LEGAL sense that the cop cannot just cite anybody for a crime. He needs "probable cause" to issue a criminal citation.
In reality, however, you can't hit probable cause with a hammer. It isn't something you can touch. Different people will have different perceptions of it.
Moreover, in many/most states there quite possibly is no method to challenge the probable cause on a traffic ticket. You fight it at trial.
Time limits likely don't matter much. In my state there is no statute of limitations on a traffic citation. You could argue due process and prejudice to a defendant's ability to defend himself, but that would likely require the passage of a lengthy amount of time, and witnesses dying and such.
Nobody has been cited yet. You have no obligation to speak with the police and your silence should not be held against you at court. If he issues a cite for the non-denying son, lawyer-up as you have said you would, but don't argue it with the cop because: 1) he isn't going to listen and yank the cite; 2) you might say something that would damage your son's case.
I suspect the 48 hour thing is not a prohibition on the state still seeking to prosecute someone. Meeting the requirement may REQUIRE the cop to issue a cite, but that's different than providing a safe haven for drivers who might have violated the law. I looked at 75 PA CS 3345 and no where in there does it lay out a prohibition. Additionally, (a.1)(3) indicates the Rules of Criminal Procedure can be followed in the alternative. Clearly, if there is an alternative way to prosecute the event, the 48 hour thing is mostly worthless.