My son has been charged with two counts of theft over 5000 and two counts of possession of stolen property over 5000 He went joy riding with a couple of friends.The ring leader got caught and blamed it on my son.The arresting officer took the ring leaders word over the other friends and also charged my son.How can I defend this? The officer didn't even charge one of the other kids and he actually drove pone of the cars

You are not going to like to hear this, but actually the police office may "arrest" but the DA decides who to charge and can still charge after the fact. Also not everyone in a crime gets charged, often one or more may get off esp if they testify.
And also it is not what the police officer things, it is what the DA decides to do..

These are serious offenses and while I know you believe your sons story, sorry but that may or may not be the truth. It appears that at this point the DA may believe he is the ring leader of this, or at least those current crimes were his idea.

You hiring an attorney to review the evidence, see what can or can not be thrown out as evidence. And also since it appears it is guilty of at least part of it.

So the only way to "fight" it, is to hire the very best attorney you can.