Hi - my name is Kirk and I'm a Criminal
Law litigation attorney. Thanks for using JA! I'm sorry for your son's trouble, but I will be glad to assist you.
A warrantless search is possible in a few different situations: (1) search incident to a lawful arrest, (2) stop and frisk, (3) Plain View, (4) Automobile Exception, (5) Emergency/Hot Pursuit and (6) consent. Here's a good article on these exceptions: http://nationalparalegal.edu/conlawcrimproc_public/protectionfromsearches&seizures/extowarrantreq.asp
It may be that the officer is claiming (1), (3), (5) or even (6) as the reason for the search. It would be hard to say at this point what the officer claims AND it would be hard to say whether or not the search was legal. Your son certainly could challenge the search and claim that it was illegal.
As for the Miranda Rights
not being given to your son, that means that anything he said while under arrest could not be used against him - - but it doesn't mean that the arrest was illegal. This is a common misconception with the purpose/reason for Miranda Warnings.
If a person is in custody (deprived of his freedom in any significant way - being handcuffed, placed in the police car, not allowed to leave), the police must read the person his Miranda rights if they want to question the person and use the answers as evidence at trial
Thus, a motion to suppress any statements made after being arrested is something that could be done regarding the Miranda Warning failure. Also, your son could challenge the warrantless search depending on the facts of how it occurred.
The most important thing at this point is to consult an attorney about these issues and also regarding the charges made against him.
Thanks for allowing me to assist you, and if you have any additional questions, please don't hesitate to ask.