What are my grand baby's biological dad's chances of getting custody? please help, cause i'm losing sleep.?

my grand daughter is 3. her bio dad had visitation when she was a small infant and he was ordered to pay 400 dollars child support. well, he was a weed smoker, loved the stuff... so he was ordered supervised visitation. he payed a couple of sporadic child support payments, visited her a couple of times and then we never see him again and no more child support for 3 years. my daughter has lived with us and we have been actively helping her raise this little girl. well, here's where it gets bad for us. my daughter was never a drug user, in fact, she broke it off with the baby's dad, cause of his pot use, BUT something in her life went very wrong. still today i can't figure it out.... she got involved with another bad boy, a meth user and now she's strung out BAD. we have her in rehab right now. while she's been living wild, we've had her little girl. well, the cps was called on my daughter and the cps placed her with my husband and myself... well, we didn't know this, but the state has been trying to go after the baby's bio dad for money he owes them for the baby's medicaid. i had no ideal.. well, the cps investigator informed me that the bio dad requested that the baby be placed with him and his family (he still lives at home). the cps told him no, because we are the ones that have been raising her and to take the baby he'd have to pursue it with a private lawyer in court. anyway, she told us to get a lawyer and get custody of my grand baby asap, because she didn't know what the bio dad would do and it's better that he go up against us than to go up against my daughter...when i first realized that my daughter was strung out, i did reach out to the biological dad to help me maybe get joint custody of the baby. he stopped communicating me and acted like he didn't want involved. i was desperate, because she took off with the baby and i feared for her life. well, the cps got involved and placed the baby with me. the bio dad owes more than 20k in court ordered child support, owes the state plus, he was MIA for 3 years. now he wants custody. what are his chances of taking this baby from us?