A nephew is a close enough type of relative that you don't have to be a legal guardian to claim him as a dependent. Based on the fact that he lived in your home more than 183 nights, was under age 19 on December 31, 2023, and did not provide more than half own support (considering the year as a whole), you may claim him as a dependent this year.
Note that if his parent also claims him, or he claims that "no one else can claim me as a dependent", and one of those person's e-files first, you will probably be blocked from e-filing. To proceed with the dependent claim, print and mail your return. The IRS will sort it out by investigating all sides of the conflicting claims.
