Each child is different & special in his own way. I believe that your son has attended & passed the preschool as for now. As other doctor has mentioned children might not take keen interest in reading or studying all of a sudden however if he gets impatient & starts crying when asked to study, you need to find out what exactly is happening at his end. Be patient with him, watch his actions & emotions thoroughly and see if it's a difficulty for him to study in some way or lack of interest. He would express in his own way if you pay attention. In either cases he can be helped to adapt to expectations after recognizing his strengths & weaknesses. Consult a child psychologist in case you find it difficult to know the reason or to manage it.