Hi Matthew,
I am so sorry to read this, especially for your little guy. We've been through similar situations with a couple of attractions, too, and it is so tough to be able to see the new, shiny attraction, but not to be able to enjoy it.
When we are at WDW over the holidays, the area where the new Dumbo attraction is located was behind a wall, so maybe it will help that if it is still that way he won't be able to see it: out of sight out of mind. With all the other attractions and excitement of the parks, maybe he won't even think about it not being available.
If it happens to be visible, there is always the chance that they'll begin a soft opening of the ride where it is open in little phases for guests to try out (however, no guarantees). We had this happen to us at Disneyland, and it was a great treat to be able to ride a new attraction before it had officially opened.
If it helps as a consolation, there are two other rides that are nearly identical the Dumbo ride: the
Magic Carpets of Aladdin at the Magic Kingdom and
TriceraTop Spin at the Animal Kingdom. You never know; he may enjoy those more!
Please let us know if you have any more questions.