Ex's dad was alcoholic. Will my son have a chance of it even if they never met?

Chance, not destiny. Alcoholism has many complex causes. Genetics certainly isn't destiny. If you son's grandfather was alcoholic, your son may have some increased chance of it. There is no specific test for this, just a risk factor. When he is old enough, it is appropriate to tell him that this problem may run in his family, same as any other medical history on either side of family (heart dz, cancer, diabetes, etc).