My brother is in prison and suffers from cerebral palsy. Are there doctors that would evaluate him for free? We need to prove that due to his condition, he is not a threat to society.

Doubt it. Cerebral palsy is a condition that varies from so mild it is never notices to so handicapping the person cannot move. Penal systems usually only provide medical care when the need arises. If your brother was mobile enough to get in legal trouble, he could do it again. If his condition is so involved, his lawyers should have pointed it out before sentencing, but could do so now.
Yes. Although there are doctors who look after inmates in prison, these doctors may not be able to assess if your brother would be a threat to society. That would likely be a conflict of interest. He should be seen by a psychologist to properly assess this.
It . It is crucial that you work with your lawyers to promote this defense. Also, there are doctors in the prison that can assess your brother, and should be assessing him regularly due to his condition. Perhaps your lawyer can identify a doctor that can speak on behalf of your brother's condition.