He is not inadmissible because he did not overstay by 180 days or more, but you're going to have a huge problem proving that you do not plan to overstay this time around. His past overstay will show up when he meets the border official, who has the authority to deny him entry into the US. Make sure you bring plenty of documentation showing that he plans to return (flight info, evidence of a job and home abroad).
In the future, if you need to stay in the US longer than authorized, you should apply for an extension of the visa with USCIS. If you had submitted evidence of your pregnancy, he probably would have gotten his visa extended without a problem!
www.gassonlaw.com - Disclaimer: This a general answer to your legal question. Unless you have a signed engagement letter with me, you should not consider this information to be legal advice.
Prior counsel is spot on. If you're eligible to naturalize then get it done asap. This would allow you to file an immediate relative petition for him and he could process through the consulate and obtain his lawful permanent resident status.
Yes, if he had a visa, it is now voided.
The above is intended only as general information, and does not constitute legal advice. You must speak with an attorney to discuss your individual case.