He does qualify as a dependent of my client. At the very least he is an "Other Dependent".

The issue is if, under legal guardianship, he qualifies as a Qualifying Child or any other classification that allows the EIP and Child Tax Credit to go to my client.

I believe there is a legal distinction between Foster Child and a child under guardianship.

I did not see any language in your links discussing a child who is under "guardianship".