I am a disabled father that receives disability and have sole custody of my son. I get a PA (Pennsylvania) ssd check from the state from him as well. I am curious if I can get child support from the mother who has not paid a dime in over a year. She use to receive the ssi for him from me when she had custody but she moved out of state and he decided to move in with me.

Your son can't "decide" to move in with you. If the mother had been awarded custody, then you had to go back to court to modify the custody order to allow him to move in with you. Only a court can change custody.

If you have done so and now you want child support from the non custodial parent, you have to petition the court for a support order.