Does he have toys that are small that he likes?
Start with tangible things he can relate to , be it erasers, pencils, grapes, toys.
Use them to teach operations (addition, subtraction, multiplication, division) using the people in his life.
Mum has 2 erasers and she gave 1 to Dad who has 3. How many do they each have? (This teaches addition and subtraction)
If Mum and dad decides to share their erasers, how many will each have? (This teaches division).
Use relatable scenarios. It will stay with him then.
If he has no interest in the subject, dont go into assessment books. I think it may bore him and daunt him even more. Already he finds it challenging to grasp, then he is heaped on even more challenging questions and one page of numbers. It can be discouraging. Introduce when he is more competent.
Finally encourage him that everyone starts somewhere and you are here to aid him. 🙂