A great aunt is considered "closely related". The child is your "Qualifying Child" dependent, because he has lived with you more than half the year. You may claim the Child Tax Credit and the earned Income Credit, based on that child (if otherwise qualified).
The only conflict would be if he also lived with one of his parents for more than half the year. That does not appear to be the case since you say "neither the custodial parent or the non custodial live with me".
You do not need legal guardianship to claim him, since you are closely related.
.A child closely related to a taxpayer can be a “Qualifying Child (QC)” dependent, regardless of the child's income, if:
1. He is under age 19, or under 24 if a full time student for at least 5 months of the year, or is totally & permanently disabled
2. He did not provide more than 1/2 his own support
3. He lived with the relative (including temporary absences) for more than half the year
4. He is younger than the relative (not applicable for a disabled child)
5. If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child (this essentially means that you have
the parent’s permission to claim the child, if the child also lived with the parent more than half the year)
6. If the parents of a child can claim the child as a qualifying child but no parent so claims the child, no one else can claim the child as a qualifying child unless that person's adjusted gross income (AGI) is higher than the highest AGI of any of the child's parents who can claim the child.
https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Ret...
