I think, first of all, that you are doing an amazing job in uncharted territory. This small human is not an easy one to raise, but your close observation will help a lot.
Second, the inconsistency between his abilities when watched vs. off-guard suggests it's not a physical issue (as Dude said). To me, especially given his age and the level of philosophical thought you've described before, it sounds like he's holding back in some areas to avoid independence and keep a close, age-appropriate relationship with his mommy. He may not truly believe you will still cuddle and read to him if he reads himself, for instance. He wants to keep a division-of-labor in his daily life that you do part of. He might worry that once he demonstrates an ability, he will have to do it perfectly all the time. He's able to understand the truth - that moving on from a toddler's independence level is a major change, and irreversible. That's going to be scary.
If I'm right, then these thoughts are having a significant life impact and are worth addressing. Mother's Day might be a perfect time to demonstrate that mothers stay in our lives for all our lives. Have a wonderful one!
