I have ADD and our three boys have ADHD (currently ages 12, 10, 4). It's always a party at our house. LOL We go to WDW every other year (but are going this year after going last year... I just can't wait that long!).
-I couldn't possibly disagree with trying to keep things as close to the home routine as possible

... but at the same time, manage your expectations at actually being able to implement any more than a few components of your home routine. Every single thing is different. To that end, flexibility and preparation are the biggest keys to limiting the pitfalls.
-I completely agree with bringing the "can't live without" objects from home. We give each kid a backpack and they can fill it with whatever they want (except loose legos-imagine that on a plane). Special stuffed animals and favorite books ALWAYS come with us. If there is something you know he depends on for any kind of comfort, if it's at all possible, bring it. It's not even worth considering what it would be like without them.
-I also agree with allowing access to whatever handheld electronics they're interested in, much more liberally than you would at home. Standing in lines, waiting in a queue, waiting for food to be served... yes-any time except when walking! At 13 he's probably not likely to be entertained by coloring or small toys... so do what works. Bring earbuds for him too. If it enables him to remain calm and endure the intolerable boringness of standing in line or waiting, without disturbing others AND without a meltdown, DO IT!

Anyone who criticizes, doesn't get it. Ignore them.
-As another way to get us through the lines, I got the Hidden Mickey book. (If you're not familiar, the Disney Imagineers have placed the Mickey head silhouette within the architecture, artwork and design of the parks, restaurants and resorts, in less-than-obvious places.) We bring the book with us everywhere and have a lot of fun trying to find the Hidden Mickeys that are in the ride queues and read up on the Mickeys hidden in the ride we're about to get on. The author has built a game into the search, with points awarded for each Mickey-making it a competition might be fun and keep him engaged and occupied. You can even challenge him to use his phone/device to make it like a photo scavenger hunt for hidden mickeys. There is another book, for other "Hidden" Treasures in the architecture and scenery, but I found it harder with my kids at their younger ages. Depending on your DS's interests, this might be good for him. I got both books at Barnes & Noble.
-Again, hard to argue with the value of a touring plan that minimizes wait times. At the same time, however, the ADHD kid can be rigid, especially when they get an idea in their head. At 13, you might be able to head off any issues by having him participate in making the plan-if he's choosing what to ride when (using the right information for wait times etc.), he's more likely to be able to stick with it.
But, we never know, do we! To that end, be prepared to skip a ride he says he doesn't want to go on (no matter how much you know he will love it), and to ride the same rides multiple times if he wishes. (can't tell you how many times we ride the People Mover!!! But haven't been on Tower of Terror since 2007) You can offer the alternative, give a reason why you think it's a good idea and/or possible consequences (such as, we won't have time to ride X ride if we ride this again), but if he still protests, consider going with the flow. Ultimately, you might miss a few rides you really wanted to go on, but you won't regret knowing that he had the best time possible because he got to make choices and really enjoyed what he did. That's what you'll remember most.
Regarding Meds... I am on Vyvanse, it's my best friend. LOL We tried DS2 on it (he was the first of all of us to be diagnosed, at age 7), and the violent outbursts were shocking and disturbing. Concerta hit the mark for him, as well as for DS1.
DS2 continued to have a few scattered outbursts (though nothing quite so violent) after starting Concerta. He struggled with appetite and sleeplessness as well. We added the low dose of non-stimulant Intuniv to his Concerta and it helped tremendously. It does not help his hyperactivity or focus once the Concerta wears off, but the outbursts have been gone for a long time, and sleep is MUCH better (without it, he's up until 3am!).
The hit-or-miss process can be unnerving, but it's not abnormal for the first medication(s) and/or dose to not be the right fit. Keep trying until you find what works.