All the information on immigration can be found on www.uscis.gov with all the accompanying forms you may need.
here is the what the US dept of state says regarding renunciation of citizenship. http://www.uscis.gov/files/nativedocuments/Citizenship_2004.pdf
"renunciation, or expatriation, is the most unequivocal way in which a person can relinquish US citizenship. The Immigration and Nationality Act (INA) provides for persons to voluntarily relinquish citizenship by “making a formal renunciation of nationality before a diplomatic or consular office of the United States in a foreign state, in such form as may be prescribed by the Secretary of State. A person wishing to renounce his or her US citizenship must (1) appear in person before a US consular or diplomatic officer, (2) be in a foreign country (normally at a US Embassy or Consulate); and (3) sign an oath of renunaction. Renunciations that do not meet these conditions have no legal effect. Therefore, Americans cannot effectively renounce their citizenship by mail, through an agent, or while in the United States.
Parents cannot renounce US citizenship on behalf of their minor children. Before an oath of renunciation will be administered under the INA, a person under the age of 18 must convince a US diplomatic or consular officer that he or she fully understands the nature and consequences of the oath of renunciation and is voluntarily seeking to renounce his or her US citizenship. US common law establishes an age limit of 14 and under which a child’s understanding must be established by substantial evidence.”
So ask your husband if he remebers if during the process since he was 16 how he renounced his citizenship. who did he renounce it too? was it to canadian officials only? did they ask him if he understood the consequences? if only to canadian officials and if he was not questioned by any american officials regarding consequences and he did not sign a form at the consulate, he is still a citizen.
the only other thing you need to ask is if he ever joined the Canadian army and if he ever worked for the canadian government which required him to take an oath. those may have been deemed a renounciation but you may be able argue against it.