If you find out that your son drinks a lot and you are afraid that he might become an alcoholic you need to take some steps to help him before he becomes addicted and then it becomes harder to quit.
Speak to him clearly and calmly; let him see the reasons why getting addicted to drinking will be bad for him, help him seek for medical help from a specialist and encourage him to join specific therapy group that will be beneficial to him. Do not yell at him, do not threaten or abuse him. Get him help before it cause more problems in his life.