Do the numbers your son describes stay constant? For example, is 51 *always* rice? Is 100 always a boy & 17 a girl? Do the numbers have "personalities"?
If not, then it could be an ASD-type behavior, an OCD behavior, or even just a particularly strong gifted quirk. It's hard to tell, but good that your pedi is aware & is watching to see how he develops.
If the numbers are personified & those "personalities" remain consistent, then there is only one place I have heard of that - a documentary on a little girl named Joni Schofield, who has childhood-onset schizophrenia. Please do NOT let that alarm you - I have no reason to suspect that diagnosis for your son & am certainly no expert. It just popped into my mind as I read your description & I wanted to point it out in case it is an area of interest. The phenomenon is described below:
"One of the primary symptoms of schizophrenia is the perception and personalization of connections and patterns that do not exist in reality. People tend to make sense of reality and perceive patterns through filters, one of which is numbers. They're the building blocks of pattern recognition and they are also ubiquitous. It's pretty easy to see patterns in numbers that are not there, even if your brain does not have a predisposition towards doing so."
