Neither you nor your son has to go down to the police. If they believe this 8-year old, the police can arrest your son, and they will, whether he speaks to them about this or not.
Generally, however, when they call the suspect and ask him to come in and answer questions, it's because they are not sure whether he committed a crime or not, and they are looking to trick him into giving them enough evidence to make the arrest.
Police are skillful interrogators and they know how to twist a person's words and testimony to turn them into admissions or confessions which can provide the basis for an arrest and then be used against him in court. If they read him his rights, they are basically telling him they intend to arrest him. Your son has a Constitutional right under the 5th Amendment not to give evidence against himself. He needs to exercise that right. His right to remain silent can never be used against him in court. Anything else he says can be.
There is only one way your son should talk to the police, and that is if he goes to see them with a lawyer. Do not let him talk to the police without one. This is a fundamental principle of criminal law: suspects should not speak to the police. They cannot usually help themselves and they can always hurt.
As for the fact that there's no evidence, that's not true. The words of the 8-year old ARE evidence, and at trial, a jury would be told that if they believed those words beyond a reasonable doubt, they can convict your son of a sex offense.
That said, there's a very different standard between what's needed to arrest someone and what's needed to convict. To make an arrest, the state just needs a reasonable belief that your son may have committed a crime against this 8 year old. The words of a believable complainant, even if later on down the road she is exposed at trial as a liar, can be enough to get someone arrested and prosecuted. It takes very little evidence to get the ball rolling in criminal court.
From there, however, the state has to prove its case beyond a reasonable doubt to get a conviction. That requires a great deal of evidence. In fact, the DA's burden of proof at trial is the heaviest burden in all law. And this is where the evidence that you have mentioned would come out in support of your son. And if it creates any reasonable doubt in the minds of the jury as to what happened, he would have to be acquitted.
So, find him a local lawyer, and do not let him talk to the police unless that criminal lawyer is willing to go there with him. Odds are, the lawyer will tell him not to speak to the police at all.