I haven't seen my son in almost 4 years. do i have rights? i'm 22 and live w/ my parents.?

he will soon be 4. when the baby was born my parents convinced me to get a lawyer to seek visitation and maybe joint custody. my son's mother, my then gf and i had broken up. i'll admit, i was a pot head and i still smoke and drink from time to time. well, my then gf got a lawyer and the judge ordered me to pay 400 a month child support and made me have supervised visitation from my weed smoking. i didn't pay the child support. it was too high and the supervised visitations were such a hastle. my ex's parents were real son of a b1+ches. i just stopped going for visits. my ex's parents have pretty much been raising my son. they're like his parents really. well, a buddy of mine told me that my ex gf has a bad drug issue.in fact, her parents put her in rehab. the cps placed my son with the grand parents. well, the human services is on my *** about child support, so i called the cps and asked them if i could have the baby. they told me no, because he's been with his grand parents all this time, they've raised him and told me that i'd have to get a lawyer... about 5 months ago my ex gf's mother contacted me and told me that my son's mom was doing drugs real bad and was with a really bad guy. basically, she wanted to know if maybe them and me could maybe get some sort of joint custody of the baby and force the daughter to leave this bad guy and go to rehab. well, i stopped communicating with her until recently when i found out that my ex is in rehab. i contaced cps, but they won't let me have my son. do i have rights? i owe like thousands in child support. with that hurt me? can i use my ex's mom's messages to me against her if i take them to court? what's my chances here? my ex's mom contacted me and told me that if i would sign my rights away that they will pay the state and they will wipe the slate of the 25k i owe in back child support payments to my ex and my son..she told me that i should have gotten back with her when she first contacted me, but now it's too late and they don't want me around the baby. what should i do? thanks