My question involves paternity law for the State of: New Hampshire

My ex-wife has a son who I thought was mine when she had him. Turns out, he is not mine. I never suspected until my current girlfriend asked about his eye color. Turns out, she was right, kid is not mine. I have very limited contact with the child and no real connection with him as he is autistic and does not really make emotional connections. His mother left the house when he was about 3 months old and the reason for divorce is "abandonment" on her part.

Can I still disestablish paternity and stop paying child support? What is the age limitation. I have contacted a lawyer, but he cannot see me until next Friday and I am too anxious to wait until then to know.