First of all, it is really wrong for a child to lie to you, but he hides his grades, which at least shows that he still cares about his grades. I know some students who are mixed with society, and I never care how much I take, nor how much I take at home. This is really not suitable for learning.
According to your account, I believe your child is not too old. As a college student, he is an experienced person in today's education system. I hope you can give him more encouragement, let him taste the sweetness of doing well in the exam, make him proud and enter a virtuous circle. If you criticize him blindly, he may fall into a disgusting cycle and imagine students more and more.
The best way to deal with it is to make him feel ashamed and let him feel from the bottom of his heart that he must do better next time. ......
Sorry, it's not the client's problem. You have to think about specific measures.