Thanks again for everyone's input! You've all been very helpful. Melissa - getting it in writing is what I intend to do. It's the only safe way to make sure I have the proper documentation.
PapaBill - the boy's father is not unknown, we just don't know where he is. Long story short - he is named on the birth certificate, but was never married to the boy's mother. He has never met the boy (now 15), nor does he care to. He did not respond to a legal petition to have the boy's last name legally changed to my boyfriend's name (my boyfriend was married to the child's mother at the time). My boyfriend married the mother when the boy was four, and continues to raise him since their divorce two years ago. Even if we do locate the father, I hold no hope that we can get him to respond let alone go to the trouble of filling out a travel consent form and having it notarized. It will be hard to prove that the father was even contacted if we cannot get a written response of some kind from him. This, in turn, could impact our trip.
Just a word of warning to those traveling with minor children - we literally stumbled across this need to have a travel consent form from the "absent" parent quite by accident. There was a typo on one of the names on our cruise reservation paperwork and I called our online TA to have it corrected. That's when I learned about this consent form. Had the name not been misspelled, I would have never known about this! While I agree that this matter should not be settled or decided in this forum, I do think it's important that single parents, grandparents, etc. know about this rule. It is to protect children against abduction - and I can't argue with that, but it does make it difficult for those who have "special" family circumstances. Divorce and death of a parent or adoption can be easily proven with legal documents, but the absent parent can be due to a whole lot of reasons. I found a website that listed problems such as "I don't know who the father of my child is", "the absent parent is abusive and I don't want to contact him for the safety of my child", "I contacted my ex-husband and he said he would sign the consent form if I made a deal with him with regards to back child support he owed". The list of problems with getting consent went way beyond anything I could have imagined.
It is unfortunate that a much anticipated family vacation has now become a paperwork nightmare, but I DO understand the reason for the consent form, so I forge ahead with optimism. I will resolve this matter directly with Carnival to make sure that we have all the proper documentation needed and report back with my findings so that others will know. I would also recommend that everyone traveling with minor children contact the cruise line AND the airline (if applicable) to get the correct information for YOUR specific situation. Thanks again to everyone for their input.