Welcome aboard, Misty! It sounds like you have a really exciting vacation coming up! Your nine-year old is in for all sorts of fun and excitement in the Youth Clubs, around the ship with you both, and in the various ports of call!
Since your husband has legal custody of your step-son, there should be no problem in taking him on the cruise. Normally, in situations where one parent is traveling with a child without the other parent, it's not a bad idea to have the parent who will not be attending fill out the
Minor Authorization Form just in case. However, since you may not be able to reach his mother, it doesn't sound like that is an option. This form is technically for adults traveling with children of whom they are not the legal guardian, like grandparents taking their grandkids, for example.
I traveled last year on the Magic with just my son, as my husband and daughter were back home. I did not need my husband to sign the form, and we had no issues at all, as I am his legal parent, listed on his birth certificate, and traveling with his passport.
You may just double check with your travel agent or the agents at Disney Cruise Line at 800-951-3532 to be sure about your particular situation, but it doesn't sound like there will be any issue.
Have fun planning this exciting trip, Misty!