A stepchild is treated the same as a child under the tax law. If you all live together then his stepchild is the same as his own child and would be a dependent if the other conditions are met. The father could only claim the child if you released the exemption to him with a signed 8332 form, but he could not claim EIC, or child care credit - only the parent where the child actually lived more than half the year can claim those.
Also there is no reason not to file jointly this year also. Filing jointly is usually always the best way to file even if only one spouse worked.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
