1. Verify with the father (the custodial parent) that he is not claiming the child.
2. Get a signed form 8332, from the father, allowing you to claim the child.
You said "His dad can’t claim because he doesn’t have a job that takes taxes out".
That's not true. There's more to it than that. It depends on what other income he has. Do they live with some other relative (e.g. child's grandparent). That other relative is more likely, than you, to be able to claim the child.
You said "I just did that tool thing you sent me and it says I can claim him on my taxes because I still buy and support him and nobody else can claim him."
That's not true. It's more complicated than that. There are two types of dependents, "Qualifying Children"(QC) and Other ("Qualifying Relative" in IRS parlance). Only a QC qualifies a taxpayer for the the Child Tax Credit. They are interrelated but the rules are different for each.
The support test is different for each type. The support test, for a QC, is only that the child didn't provide more than half his own support. The support test for a Qualifying Relative (QR) is that the taxpayer provided more than half the relative's support. If you claim the child under the qualifying relative rules, you only get the $500 Other dependent Credit, and not the (up to) $2000 child tax credit. You want to claim the child under the rules for divorced and separated (including never married) parents, not the support rule (you want him to be a QC, not a QR).
As others have said, when the non custodial parent is claiming the child, you cannot claim the Earned Income credit (only the custodial parent is allowed the EIC). But, you can claim the Child Tax credit.
