It's all normal for your son. This is who he is.
When you meet another little one, watch to see what is normal for them. Every kid is different, and few will be like your son in the ways you described.
I perceive letters and numbers the same backwards and upside down. I can tell they aren't forwards, but it takes very little more effort for me to read mirror writing or a page someone across the table from me is looking at. Some people are just wired that way. It hasn't caused me any problems, that I know of.
Yes. This. DD and I both do this, as well-- not so much with numbers (and me even less so than her with digits), but with text, absolutely.
DD knew both upper/lower-case alphabet and phonemes by 20 mo, too. I'm not really sure how she learned them. Actually, she knew the letters by 18mo, and I know that because that is when we moved 3K miles, so I'm very sure.
Does the arc slow?
Well, it depends on what you mean.
Does it begin to resemble something more normative?
NO. It does not.
Does it begin to resemble something which would be normative for a child X months/years older than your own child? That is, can it be understood as a sort of-- tunneling phenomenon where your 2yo has "tunneled" into being a 5yo?
No again. Your child is still 2 physically, and maybe emotionally... and maybe in some ways even cognitively.
What that arc looks like
is both:
idiosyncratic and even unique, and
asynchronous.That means that from here forward in life-- and maybe life-long, actually-- your child is going to be out-of-sync with agemates AND with those who are peers in particular domains of development or interest.
This is a hard thing to live with for children. It's what makes gifted children "special" kids as much as those with developmental disability.
The world at large isn't built for asynchrony-- which means that your child moves through a world that is various degrees of ill-fitting or uncomfortable, and may well feel that
he is "wrong" or that something is wrong with HIM-- which obviously isn't the case. He is who he is.
The most enjoyable things that we've gotten to experience on our journey as parents to a child like this are deeply private, quirky, and difficult to explain to outsiders.
Many are "you kind of had to be there" things, like her sense of humor at various ages (what other five year old "gets" Monty Python and uses quotes, chortling in that little-kid way about it all??), her insights which are informed by a highly idiosyncratic world view (Romeo and Juliet is a
lot darker when you don't assume that "young love" is at the bottom of Juliet's actions, let me just say). The recognition of patterns in places/ways that adults have stopped "seeing" them. Connections between adult themes or works of literature/art to materials intended for children-- made possible because of the compressed timeline that means that for her,
The Velveteen Rabbit and
Great Expectations or
Pericles do "go together."
The way that dirt swirls in a stirred mudpuddle DOES look like a fractal pattern...
That kind of thing has a quirky and intense magic all its own.
ETA:
The 'black box' learner thing is something that we've come to expect about DD, now 15, nearly 16. She doesn't "learn" so much as comes to master something via some internal process that involves stirring, and back-burner simmering until....
VOILA!Mastery.
She
claims (recently) that she never "learned" how to read, how to do math up through trigonometry, or to master physics or literature analysis. That she was sort of born knowing HOW to do those things, or picked them up osmotically over time, to the point that they all seemed rather obvious once the time came to 'display' her mastery of those things. If that makes sense.
She also has the stubborn streak of being "unteachable." It can be quite maddening to instruct her-- she doesn't make
progress in any discernable way, generally speaking-- it either IS or it ISN'T.
We call this phenomenon "quantum learning." Don't know if that is the right term for it or not, but there is no other emotionally resonant way to describe just how shocking/jarring the phenomenon is.
She has
always been like this. It's how she learned to walk, to talk, to ride a bicycle, tie, her shoes, to read, etc. etc.
It's as though the ability is inside her somewhere, and suddenly she breaks through and can ACCESS it.