I'll admit that this was upsetting to read--your son is literally beating himself up over this. And I would guess his inner monologue is probably at least as equally unkind.
There's a lot to unpack from your comment to which I'd like to respond but given how long it's taken me to even sit down to write this, I will limit myself to urging you to read "Driven to Distraction: Recognizing and Coping with Attention Deficit Disorder from Childhood Through Adulthood" by Edward M. Hallowell & John J. Ratey (https://www.goodreads.com/book/show/108593.Driven_to_Distrac...).
One of the reasons why I recommend this book is that it is very focused (you may wish to excuse the pun) on establishing a correct diagnosis before moving onto possible therapeutic approaches. This includes a chapter on issues that are misdiagnosed as ADHD/ADD when the cause may be something else and another chapter on misdiagnosed issues which may actually be ADHD/ADD.
The book also addresses a wide range of other aspects related to ADHD/ADD which often aren't mentioned:
* Mood swings
* Difficulty communicating "serially"
* Acceptance of diagnosis
* Heightened sensitivity to criticism
* Parental guilt over existence or non-diagnosis
* Mourning over lost opportunities/time
* Impact on relationships with family, partners, friends & authorities
It also includes case studies of individuals, families & partners and how they have been impacted.
FWIW a common refrain from adults with ADHD/ADD is that frequently the biggest/longest-lasting damage that un-diagnosed ADHD/ADD has on an individual is the anxiety, depression & self-hatred that comes from not being able to do the "right thing" and not understanding why. Something exacerbated when others around them are all too quick to reinforce the message that it's because they're lazy, lack willpower, or, are just not trying hard enough.
On the other hand, unlike many other mental heath disorders, even just having a diagnosis can actually be helpful because it provides some answers to "why am I like this" and a new lens through which to view oneself & interactions with the world--and hopefully also leading to new ways to cope & progress.
Personally, I feel it's also worth contemplating why you (and others) were/are skeptical about the existence/reality of ADHD/ADD.
My observation is that a contributing factor to skepticism is that ADHD/ADD intersects with an almost perfect storm of societal/cultural fears, including:
* Fear of children being negatively affected by misguided/incorrect parental actions.
* Fear of the impact of "unnatural" medication.
* Fear of medication that can be misused by those not affected by the disorder (and lack of understanding on how the medications work for individuals with the disorder).
* Fear of misdiagnosis due to the "invisible" nature of the disorder.
* Fear of not applying discipline & boundaries in childhood negatively impacting the child's future
* Fear of enabling bad character traits such as "lack of willpower" & "laziness" by accepting an "excuse" for behavior rather than demanding more effort. (And other effects of a "puritanical" belief that the solution to every problem is to "work harder".)
* Fear of someone else gaining advantages/shirking responsibility through deception.
It's also worth noting that--like in relation to many medical issues but exacerbated by ADHD/ADD even triggering skepticism in the medical community itself--the response/assistance you & he will receive regarding an assessment will unfortunately be highly dependent on the individual medical practitioner.
I'm just a stranger on the internet but for the sake of your son please strongly consider at least reading the book I suggested and preferably seek an assessment from a medical professional with experience in such disorders.
Oh, also, you don't mention how old he is but perhaps when a similar restaurant situation happens again (or even now) you could listen to his answers to some gentle questions from you about what he thinks just happened and why it happened. It might give you some insight into his experience of the situation/existence.
Additionally, I do think it's important to acknowledge issues like this aren't easy for child or parent, so I consider it positive that you shared your situation & asked for recommendations.
Thanks for taking the time to read and consider my suggestions.