Or is this routine enough that a public defender will be satisfactory?
In general, for stuff like this, public defenders are every bit as good as a private criminal defense attorney attorney you'd hire. Just like private attorneys, some will be better than others, but on the whole both groups of attorneys provide competent representation. If your son doesn't have the money to hire a private attorney, then he doesn't have much choice in any event.
Arraignment is Thursday. Should he seek representation before arraignment?
He can ask. However, in some places the PD is only appointed after the arraignment.
Does he enter one plea for everything or plea to separate charges.
He'll be asked for his plea on each charge. In the absence of any legal advice before the arraignment, he'd want to enter a not guilty plea to each to keep his options open.
Charges then are: Possession 4 oz. (proveable);
Just from what you've said here, the state would seem to have a very good case on the possession charge.
DUI (looks shaky to me but I do not know)
The cop smelled pot in the car, suggesting your son had been smoking it before driving. If your son's driving was such that it indicated impairment, that is enough to support the DUI charge. DUI includes driving under the influence of drugs as well as alcohol. If the tests they run come back positive for pot, expect that the state will have a pretty good case on this, too.
Possession of Controlled Substances (they are his prescription meds, nothing else - they are anti-psychotics - he's under treatment for biopolr but doing well);
He is supposed to keep his prescription meds in the labeled bottle in which they were dispensed. That said, if he can prove that the drugs found were drugs for which he had a proper prescription, the DA may let that go.
and No Proofe of INsurance (he's insured, just no proof with him).
This will depend on the applicable state law, and I've not looked up Kentucky law on this. Some states make it an offense not to carry the insurance card, even if you have valid insurance. If Kentucky has that kind of law, then he may still be guilty of that even if he proves the car was insured.
He was not Mirandized but no formal interreogatoion took place.
The Miranda advisement is only required if the suspect is BOTH (1) in custody AND (2) being interrogated by the police. If the cops didn't ask him anything after arrest, the advisement was not required. Even when required, the only effect of a failure to give the advisement is that the statements made by the suspect may be suppressed and thus not used against him at trial. It does not require that the charges get dismissed. If the state has enough evidence other than the suppressed statements, then the prosecution can go foward based on that other evidence.