To the OP, I'm certain he'd be able to order a dessert of the kids' menu at least 99% of the time, if he wants to. Personally, I don't think the build your own sundae is as exciting as it sounds, especially for older kids. A scoop or two of vanilla ice cream with little cups of chocolate sauce, sprinkles and maybe some M&M's or crushed cookies. Then again, my 9 yo never got tired of it--she even kept ordering it at restaurants where she had other options. The point is, though, that he'll be able to make the choice.
My husband did this very thing at the Concourse Steakhouse when we were there last month. He had eaten his dinner, and was full but still wantd something sweet for dessert, and the one scoop of ice cream witht he chocolate sauce was perfect, so he just asked the waitress if it was possible, and she said sure and brought it for him. Sometimes you don't want another fancy dessert, and a simple old-standby is great! Have fun, I am sure your son will love trying many new things!
You shouldn't have a problem, as your child is considered an adult by Disney Dining standards, so you are not trying to get an adults dessert for a child. However, you may have to explain this to your server. We had the opposite situattion at Sci-Fi (my Dd, 10 and considered an adult) wanted to order a kids meal but an adult dessert. The server said no. I asked why not, as she was actually saving them money by having a kids meal, and not the other way around. He got all flustered and said he would have to speak with his manager. He came back a few minutes later and said THE MANAGER said it was okay... leaving us with the impression that he (the server) still didn't. Whatever
). A few times times I ordered what she wanted and "traded" meals with her. Again, no one cared.
, not silly at all! 
