Can I claim a nephew who is living with me and I am supporting without a legal guardian document?

He has lived with us since March 2021. His mother has left and I can't contact her. I do not know if she has received any of the advanced child tax credit for him. We signed him up for Medicaid in his own name until we have legal guardianship to add him to our insurance. He is nine years old and the school has him listed in the Federal McKinney-Vento Act for homeless students, but he does not receive any direct funds for that, it relates to school activities only. We expect the court to award guardianship in early 2022, but we are unsure if we can claim him in 2021.