If his mother is in jail, where is your son living? If he's living with you and you already have joint custody, you need to consult a Texas matrimonial lawyer. Whether or not you can get full custody depends on a number of factors, including how long his mother is going to be in jail and what she went to jail for. In general, the courts are reluctant to take away custody from a mother unless there are aggravating factors which make her a danger to her child. But whatever it is, any change in custody requires an order from a court.
If you and your son live in the State of Texas, please feel free to call me. Perhaps I can help you or I can refer you to an attorney that I know and respect.
I only handle Harris County cases. If you live in a county close to Houston, Texas, I know of many excellent attorneys that might be able to help you. I will try to find someone based on your household income so that you can afford the attorney.
If Mom's in jail, it's easy & quick -- unless another person has been granted legal custody by a court.
You will need to file for a modification of your prior order in the court thatmade the order for your son to live with his Mom. His mom being injail will be a ground for you to get custody, but you need to hurry before the person he is staying with now has custody of him for 6 months, because then that person can file for custody and you have to fight with them.