Obviously an old old old post...but answering anyhow.
1) you don't put any his prior year (2015) tax information in the 2016 tax return you are now filing together. His 2015 data income deductions etc, must still be filed in his own 2015 tax return. You/HE buys the desktop 2015 tax software...prepare his 2015 taxes and file it by MAIL...no e-filing allowed for a long-past due tax return. Your marriage in 2016 does not affect his filing status for 2015. if he was single all of 2015, then he files single for 2015 (unless he was unmarried and had children dependents...then he can perhaps file as HOH (head of household).
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2) for anyone else in this situation, where the new spouse did not file taxes in the prior year, when e-filing current taxes, you would indicate the spouse who didn't file had an AGI of zero for the prior year's file. Thus right NOW ...if this situation was for 2018 taxes, and the new spouse didn't file 2017 taxes ...the one spouse who filed in 2017 would have an AGI to enter in the final e-filing page, and the other spouse would enter a zero for the prior year AGI (since they didn't file in 2017.....even if they caught up recently and mailed 2017 in...they would still use a zero)
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
