Importance of counting
Even if you can count from 1 to 100, it is meaningless if you don't combine numbers with real life. It's too early for children to say 3+2=5. It is ok to count from 13 to 20 before entering school.
Combined with real life
Instead of asking the child to count from 1 to 100, let him combine something with his fingers or eyes and repeat "1, 2,3 ..." until he counts to 13 or so. For children, the combination of things is of great significance.
Order of numbers
He should also be taught to count "first, second ..." and tenth in order.
With eyes and hands.
Not by calculation, but by eyes and hands, we will simply compare which is more, which is less, which is longer and which is shorter.
Count the common things
Teaching him how to count common things, such as how many people, cars and pencils are there, is much more useful than counting from 1 to 100.