I have a stepson who is an adult child. He is autistic. His father passed away from cancer and his biological mother doesn't want to pay for med needs

My name is Mariza Arko, and I am a stepmother of an adult child. My stepson, Joshua Bath, is autistic and does not have the ability to live on his own. His biological father passed away two years ago from cancer. Now, as a single mother, I cannot pay the medical expenses needed to help Joshua with his disability. Joshua's biological mother is still alive and makes a very good living as an accounting manager. She, however, refuses to pay any of the medical and coaching expenses that Joshua needs. Joshua was not declared an adult child when he was younger because his father never dreamed he would get cancer at such a young age. I seek legal council on how to receive help from Joshua's biological mother.