G.♣.
Reward him for being honest. It isn't uncommon for kids to tell lies or stretch the truth to get out of trouble. Our kids know that if they do something wrong there will be consequences. But if they lie about it, the consequences will be more severe.
You have to teach him that you will be more understanding and the consequences will be less severe if he is honest with you.
ETA - If your son is always getting in trouble at school, it is high time you had a meeting with his teacher. Find out what he is doing, why he is getting into trouble, what the teacher is doing to try to change the behavior, etc. Calling you all the time and telling you about it doesn't really help the situation. The teacher needs to deal with the problem. Find out what the plan is.