My son who is on SSDI, needs to find a lawyer he can afford in a child custody case. How do we find one we can afford and make payments to?

We tried legal aid, but they don't accept these kind of cases. He has a son who is 5 and was never married to the mother, he split up with her over 2 years ago. She was only seeing her son 4 to 8 days out of a month. We have had him now over 2 months, but my son called the police when she just showed up, because she was with a convicted child molester. We need to get legal custody. Legal aid won't take the case, but told us NOT to give her the child, so did the police, and DHS. DHS said my son since he has him all the time, can enroll him into school here, take him to the Dr. of his choice, She said it didn't matter about legal custody since he is here all the time, but others say differently. He is 5 shots behind on his immunizations, when he had a tonsilectomy, adnoids removed and tubes put in ear drums, she took him for over 24 hours and brought him back to my son without having given him any of his medications, they were still sealed. She has done this everytime he was sick. HELP!