My son has anterior cruciate ligament injury. is that something serious?

Yes: Reconstruction of an acl tear should strongly be considered in a young active individual especially if they want to continue to play sports/engage in high demand activities. Without an acl they may continue to experience giving way of the knee, each time potentially incurring further damage to the knee.

Would depend on trea: This would depend on treatment options you choose. The cruciate injury is a major debilitating injury that can be rehabbed to about 80% of pre injury status when properly evaluated and treated. Would suggest that he be seen by a specialist as soon as possible.