The IRS will not release a tax return information without owner's consent.
There is no way you may obtain your son's tax information or his place of employment from the IRS without his permission.
You may try other ways - such as white pages, property registrations (house, car, etc)
You may use a private investigator - but that might be a little expensive for your situation.
To enforce a child support his ex-wife should contact Child Support Enforcement Division - www.dhs.state.mn.us/main/id_000160 - they have legal ways to locate the debtor and enforce payments.
Let me know if you need any help.