I have a daughter about the same age, and she is going soon. My experience is that this needs to be done step by step-before learning the vertical calculation of numbers within 100, you must be very skilled in addition and subtraction within 20. Because vertical calculation is divided into carry and no carry. In addition, don't force children to study when they are emotional, which is not conducive to learning. You can take advantage of his happiness, and then promise him that if you finish these five questions (three or four of which he has mastered, which belong to improving self-confidence and laying the foundation), you will give him certain wishes (such as what to eat or play with him, or what reasonable requirements you have), so that his progress can be rapid.

I hope this helps.