My ex wife got the sole custody of my son. Recently she is cirtically sick n will not be able to regain her health within 1 year.

Right now I'm spending two days of the weekend n evening in the weeday to help ,y son to tuitor his home work and bring him to his actitvy.

He is now taken care by my ex wife parents who is unable to tuitor his school work.

Do I have a high chance to regain the custody of my son?

Yes, possibly only temporary though. I would visit your family court and request temporary custody.

Yes, if mom is sick, you should apply for temp custody. If mom recovers you can give her back custody, if not, you can make it permanent.