I think your good little boy is trying to tell you something: “Don’t push quite so hard, Mommy.” It sounds as though he is very bright and also as though you have given him plenty of encouragement to learn new things. That sort of maternal teaching is very helpful to young children. It helps set the stage for life-long learning and forms a solid foundation for the formal learning that will be expected of him in school later on.
There are at least two reasons why he may resist doing this when you ask him to show others what he knows. One is that he is shy and doesn’t like being put on the spot. The other is that he has figured out how much this means to you and can feel in control of the situation if he refuses to recite. I wouldn’t make a big issue out of it. The important thing is how he uses the learning, not how readily he demonstrates to others what he has learned.
And you are wise to single him out for special Quality Times with you. This is especially important now that he has a baby sister.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.