I've read the book but I'm not a 100% follower of it. DD13 and I have issues with carbs that are different than what the diet would allow but I do use some of the info from the diet to help with decisions for myself as well as my DH who has T2 diabetes. I have followed complicated dietary restrictions. My answer to your question will be based on this. Again, I have not actually followed SCD in real life or at WDW.
CS is going to be difficult. Extremely difficult. I don't know if there really are sides that are SCD compliant and I'm pretty sure there aren't any desserts. You'll have to get info from Special Diets about entrees such as roasted chicken or grilled salmon to find out what they're marinated in/dusted with before cooking. I would make an effort to stick with TS meals.
I know you'll have a lot of snack credits but I'd carry snacks for him rather than trying to find snacks in the parks. I don't even know that there are any SCD compliant fruits available in the parks. Your resort may be able to order in something that you can pick up from them using snack credits. This is something you'll need to discuss with Special Diets. It may take a few emails, but insist on more info/contact than the generic form emails and attachments. IMO SCD falls under the category of a special diet that they list as requiring more info. They should put you in contact with a chef at your resort who can work with you regarding what can be ordered in for you.
TS meals will have some options for entrees but I'm not positive what beyond meats they'll have for you. I wouldn't count on there being any salad dressings available for even a plain lettuce wedge unless they maybe have some red wine vinegar to go with oil. (is RWV safe? I'm pretty sure balsamic isn't but it's been a while since I looked at the details.) Many of the TS restaurants will have some vegetables that are SCD compliant and for the most part chefs can prepare these plain or with salt & pepper or maybe even cooked up with a bit of bacon if you're allowing a bit of bacon. Even if you speak ahead of time with chefs at restaurants though, don't count on them ordering in anything special to the restaurants. Over the past couple of years I've found that while chefs may say they'll try to get stuff/do stuff for us, they don't. I look over menus to see if there are options that look easy to modify to be safe for us and don't book if I don't see much. If they have ingredients then they will customize a dish for you. They do have to have the ingredients though so again, look over menus. Unfortunately, even at TS locations I wouldn't count on desserts.
Wait, I can't remember, are there any cheeses that are SCD compliant? I know milk has to be specially treated but aged cheeses may be ok. Anywhere that has cheeses should be willing to provide him with some for a dessert alternative.
Also, if I'm remembering correctly, melons are safe, aren't they? You can get cut up honeydew and cantaloupe at a variety of places including fruit carts in the parks. You'll also find whole apples. I can't recall if all apples are safe or not. At least green ones are, aren't they?
If I think of anything else, I'll let you know.
