My sons mom is strung out on drugs. can i get custody?

i guess i am asking these questions wrong cause i cant get answers. here goes again. i have not seen my son since he was an infant. he is 4. i had supervised visits but that was pure hell all because of a little weed. i owe the state child support, they are after me big time and my sons mom up to 20,000 or more. i stopped showing up for visits and i could not afford the child support payments. i found out that my ex gf my sons mom is on meth. the cps placed him with my sons grand parents, cause they have been helping raise him. i called the cps and asked if i could have him placed with me and she told me no cause my son is used to the grand parents and thats been his home since he was born. she said that i would have to sue for custody but she wasnt going to remove the child from a stable home herself. do i have a chance to get custody and will i no longer owe a sh1+ load of child support if i get him? just like to add that i am almost 23 and live at home. i recently got fired from the shoe dept and now am a cook at tia jaunas mexican food restaurant.