My son has been sentenced for a criminal crime that was commited by his cousin.Of course he is denying that he was involved.

If your son has been sentenced, then that would mean that there has been a trial and he has been found guilty based on the evidence presented.  If there was the possibility of jail time, your son should have been appointed an attorney, or you could have hired your own attorney, who would have been present at the trial.  I suggest you speak with that attorney about what you might do at this point, and what sort of evidence you have that it was the cousin who actually did the crime.  There are some time limits on what can be done so I would suggest you speak with that attorney as soon as you can.