dui on private property?

i live in edmond,ok.a few days ago, my son (age 19) and one of his friends were fishing on private property ( another of his friends ) on which they had permission. the edmond police was "investigating"possible trespassing. the police drove back to the pond where my son was in his truck about to move to another pond on the same property. the police questioned them and smelled beer and required him to take a breath test. he tested .02 which would qualify for legally intoxicated/dui for someone driving that is not of legal drinking age. at that time they searched his truck and found nothing else. they went ahead and arrested him for dui, handcuffed him and left. once they were on their way the officer that stayed behind " to wait for the wrecker as they impounded it also" called the officers transporting my son and told them to arrest him for marajuna possesion and drug paraphanalia" as his second search turned up a unused pipe and some pot in the glove box".
all that being explained, my question(s) are
1. what legal basis can an officer arrest anyone for dui on private property?
2. my son does not use drugs/pot and even offered a drug/blood test, so we feel that for whatever reason they planted it as it was not found in the glove box by any of the four officers ( 2cars of 2 police) on the initial search, but only after he was a mile or so away.
3. if, there is no legal basis for the dui, can the "evidence of the search" be considered for charges?
4. should we consider suing the city/pd for conspiracy?
5. i might add that, for good measure the police had the truck towed and impounded to another little city about 15 miles away eventhough there was two firms in our city that regularly do this at all hours, one only about 1 mile from arrest sight.
thank you for any insight

First and foremost, you have to read the statute he charged with violating. It will say whether the offence has to be committed on a "public highway." If it does, then you should be ok, for it has to be against the law to drive drunk on private property.