Jolaine83 maybe your son is testing your reactions? I'm not sure what the developmental schedule is for that, but I'm guessing it's something they can grow into after they've learned some scholastic skills (if the scholastic skills are early).
My DD10 "forgot" how to read when she started preschool (she was about 3.5 yrs at the time). I was new to this phenomenon, so I actually believed that she had forgotten. She had been reading at about a grade 2-3 level, and once she started preschool and became exposed to other kids her age, she just suddenly forgot how.. even forgot the sounds the letters made, etc etc.
I actually thought that maybe she was being so socially stimulated that the reading area of her brain was resting to compensate! Then my Dad (wise ol' grandpa

) tricked her into reading one day (pretended to read something wrong and DD corrected him). LOL-busted!
I suspect with her it was because she realized that other kids her age couldn't, and she didn't want to be different.