Disney will list his name as whatever you give them - my eldest child (still a minor) has a different legal last name than the rest of us but uses "ours" for everyday purposes. That's the name on her resort reservation and as it's not her birthday in the parks, I don't see it making a huge difference as it has never been an issue in the past.
If you're FLYING however, it could be an issue. You must book your tickets in the LEGAL name of the traveller - therefore my eldest is booked for our flight under her legal last name for identification purposes. If his legal last name will change before your flight, call your carrier and have your travel documents reissued. I had to do this for our August trip to the Carribbean, as DD #1's last name was incorrect on our travel docs (thus it was wrong on the passenger manifest and would be wrong on our boarding passes). If the name on the boarding pass does not match your ID, you will be denied boarding (or at the very least you will have a bigger hassle getting through security - this information came from an inquiry made directly to TSA).
There are new TSA regulations now regarding exact match of ID's and traveller manifests, as well as gender and birthdate requirements. The airlines are phasing these last changes in slowly but they are STRONGLY encouraging people to book passage using the name on their state issued ID's.
As the last trip I took under these new rules was out of the country, I have no idea how they will confirm the identity of minors who do not have state-issued photo ID for domestic travel. My SIL is bringing my niece's BC just in case she needs it for the airport (and she will need it anyway for Disney for the free birthday admission). I am toying with bringing the girls passports (just in case) but I'm probably going to send anothe quick e-mail to the TSA website just to ease my mind. They got back to me within 12 hours or so.
Call your carrier if you're flying and check - you still have time to get new travel docs issued if you need them. If you used a TA, they are the ones who must make the change as the reservations agent will tell you they are not able to. What the reservations agents can do is make notes all over your reservation about the name change and reason for any airline employee to see who looks. It wouldn't help with any TSA issues but you would just have to leave lots of time to make it through security before your flight. This may bring the "behind the scenes" issues to light in front of your son (which I understand you want to avoid). Your other option would be to use his "old" birth certificate if that shows his name as listed on the passenger manifest - it will make travel easier and you can think of it as a "last hurrah" for the old name.
And yes...you do have the best husband in the world. Men who will do that are few and far between.
