A little advice:
1. Suggest that he read a book. If parents have time, urge him to tell parents what the math teacher says every time.
In this way, you can be familiar with his math knowledge and help him review his math. In the first grade, you can ask him to tell you formulas and theorems, and remember them at any time, such as when children are playing or other events. Mathematics is a process of understanding. If he learns by rote, he can't, but if he understands it, he can remember it for a long time.
Parents can read math books with him if they have time and are willing to spend time. The reason is that children find mathematics boring and difficult to understand, which can help them understand. You two can watch together and discuss with each other. If you don't know, you can go online to consult or ask others, but you must understand every sentence in the textbook. It's best to do all the exercises and examples in the math book. Don't be afraid of difficulties. After this step, he will automatically get on the track. After about half a semester, he will get better at math. You don't have to worry about his math anymore.
This is actually a question of learning methods; The above methods are my personal experience. When I was in primary school, math kept slipping, but my mother trained like this, and then math gradually improved. I will always be the representative of math class, thank you!