The reason you can't diagnose adhd from hyperactive behavior in toddlers is that MOST toddlers are hyperactive. It's how they are *SUPPOSED* to be.
Now... some toddlers are super mellow... but they are the rare ones.
How you diagnose adhd in toddlers is by looking for other symptoms of the disorder that are NOT age appropriate. Like intense concentration that can last for hours ((most toddlers have an attention span of 15-20 minutes if you're lucky... adhd not only comes with the distractability and energy... but also hyperFOCUS. If it's interesting we're on it like white on rice. In toddlers for up to several hours... in adults we're talking 4-20 hours EASY (without peeing, eating, or in any other way "coming up for air" until something grabs our attention... and then our body rather irritatedly presents it's "list of complaints"... like the fact that if we don't make it to the bathroom in less than 10 seconds we're going to have an accident! The adrenalin will usually get us to the bathroom okay... but then we're talking dizziness & stars from lack of food... bruises from leaning on one part of our body for too many hours... the list goes on).))
Other less well known diagnostic criteria for adhd include things like hypervigilance, hyperawareness (usually manifests in sensory issues, both positive and negative), giftedness, multitasking (like working better/being calmer in chaos rather than quiet... a fairly well known example is that adhd school kids usually needs loud exciting music on in order to be able to focus on boring work... whereas a non-adhd kid needs quiet, or calming music played quietly), & opposite stimulant reaction. (Stimulants in small doses perk us up and focus us... but in larger doses actually put us to sleep. For boring meetings I have to sit still through, I can drink a Venti Caramel Macchiato and be mistaken for being stoned).
Movies don't count for hyperfocus, btw, as movies can zone out most humans, regardless of age. Flashing pictures and sound... they're hypnotic.
You're little boy sounds lovely, btw. But while he might be adhd... everything you described sounds like a perfectly normal, healthy, toddler. Although the reasons you gave *against* the idea that he might have adhd don't carry weight. Many adhd kids do phenomenally in school (especially in school settings where they follow their interests for the majority of the day... like montessori or waldorf or most gifted schools), since they have longer attention spans than most toddlers, and are so emotive the social thing isn't an issue until they become a "trouble maker" or aren't responding "correctly" to social cues (aka in preschool a lot of adhd kids are uber-popular), and as was already mentioned... the whole hyperfocus thing. It's not that adhd kids and adults can't pay attention... it's that we can't pay attention to anything b.o.r.i.n.g., and that we often get lost in the wild adventures going on in our own minds, or in the wonder of the world around us.