Mom of a 4yr old boy,been with his dad parents 4 8 mon dad in prison,not on birth cef. how can i get him back?

im a mothor of a wonderful 4year old baby boy who is looking for sum advice!! my son haves been staying with his fathers parents for 8 months. the father of my son is in prison. i go see my son & visit with him as much as i can. we both live in diffrent towns so its hard trying to go see him with out a car.under the stress & not geting along with my son granparent its very difficult for me.what r my right plz help....thank you so very much!