Pediatrician but not YOUR pediatrician, so take with a shaker full of salt. Some great advice already given here.
One of the additional bits of information I’d want to know is if there is any family history of learning disabilities, especially in regards to reading. That could show up only as adults who read fine now but are horrific spellers.
I’d also want to know if the observation is not having a hand preference or having a much harder time telling his right from his left than his peers.
If there is no strong family history of reading disability and it is more hand preference I would, presented with a similar story in my office, lean to the not to worry side. (Difficulty with knowing your right from your left is associated with reading disability, and reading disability has a strong familial component. Being ambidextrous is not, that I’ve heard of anyway.)
Development is NOT necessarily linear and as pointed out the expectations we currently have of reading skill level by KG entry are both recent and not predictive of future achievement. Sooner does not mean better or getting farther. One personal vignette is my own oldest (now 34 and in his psychiatry residency) who finished KG barely knowing his letters, himself upset because his friends were all readers by then. One month into first grade it just clicked and he could not only read but could at several grades higher levels, basically overnight. Not linear. More like hit a big step on a staircase.
Development is also not evenly distributed. The average child averages out to be average at any particular point but is unlikely to be average on any specific developmental channel. There is a certain amount of developmental energy they have to distribute into the different buckets, and more in one leaves less for the other.
His language and social skills being strong … more than anything else, his sense of humor and creativity being strengths … portends better than if he could name letters and numbers right now.
A study I’ve always wanted to see is one that actually codifies the humor milestones and correlates hitting those milestones with future outcomes. My bet and anecdotal experience is that it correlates better than any other track. A kid advanced in their humor milestones is one to mark for possible gifted needs later, and one delayed will more likely than not eventually need extra educational services. No matter if they know their letters and numbers early or late.
Meanwhile the what to do (I’d say, presented with a similar story in the office) is less trying to drum in the letters and numbers but keeping up the language activities, especially reading to him lots and creative play. Modelling clay is good for building fine motor strength that you use for writing.