Final answer:
Isaac is in Stage 3: Initiative vs. Guilt according to Erikson's Theory of psychosocial development.
Step-by-step explanation:
Based on Erikson's Theory of psychosocial development, Isaac would be in Stage 3: Initiative vs. Guilt. This stage occurs during preschool age (3-6 years) when children have the ability to initiate activities and assert control over their world through social interactions and play. Isaac's interest in school, reading, and conducting experiments demonstrates his sense of initiative and his desire to explore and learn.