Can my common law husband claim my son for tax credits?

No. He also cannot file as Head of Household.

You and your son may be his tax dependents, so he can claim your $4000 exemption (deduction), if you lived with him all year (see full rules below). But because you and your son are not related, your cannot be a qualifying child for the earned income credit and child tax credit and neither of you are a qualifying person for  Head of Household filing status.

A person can still be an other dependent (Qualifying relative in IRS parlance, even though they do not have to actually be related), if not a Qualifying Child, if he meets the 6 tests for claiming a dependent:

1. Closely Related OR live with you ALL year

2. His/her gross taxable income for the year must be less than $4,000 (2015)

3. You must have provided more than 1/2 his support

4. He must be a US citizen or resident of the US, Canada or Mexico

5. He must not file a joint return with his spouse or be claiming a dependent of his own

6. He must not be the qualifying child of another taxpayer

You may be able to file as Married Filing Jointly if you live in a state that recognizes common law marriage. Currently, the following jurisdictions recognize common law marriage:

 Alabama

 Colorado

 District of Columbia

 Iowa

 Kansas

 Montana

 New Hampshire

 Oklahoma

 Rhode Island

 South Carolina

 Texas

 Utah

Georgia(1997), Idaho(1996), Ohio(Oct. 1991), and Pennsylvania(2005) are grandfathered for the marriages before the year indicated. Living together in a common law state is usually insufficient - you need to hold yourself out as married including owning property together, having joint bank accounts, etc. To find out your state’s rules see:

http://family.findlaw.com/marriage/common-law-marriage-states.html

If you are able to file jointly, then you (the couple) DO have a qualifying child for the tax credits.