I won a small claims case. I have not been paid and am not able to garnish his wages due to the amount of Child Support he is already paying. How can I file a garnishment on his IRS taxes and collect my money?

Basically you can't. What you can do is attach his bank account and, if the tax refund is deposited grab it that way.

This is the Catch 22 of obtaining a judgment. Often the judgment is easy to get, collecting is another matter.