What a parent can do depends on the son's age. If he is not yet legally an adult, the parent can and should take advantage of being able to make decisions for him. Therapy, rehab programs, AA are all there to offer professional or group support help, even if it requires police assistance or commitment by a doctor to get him there. (Expect anger and resentment from him.)
If he is an adult, the parent has fewer choices--actually, almost nothing that does not require the son's voluntary compliance. The entire family may stage an intervention, which sometimes works. If he reaches the stage where he is potentially a real danger to himself or others, a parent can go through the courts to have him committed for treatment. (Expect very heightened anger and long-term resentment if you do this, but you will be guided by the situation. It is better to have your child hate you than to watch him kill himself with an addiction.)
Talking to an alcoholic who is in denial is like spitting into the wind. Not much, if anything, you say is going to stick. But mothers (and fathers) don't give up. They keep trying to get through to their kid and hoping that he will eventually come to realize he needs help to get that monkey off his back.
It may take years, and it's possible he may never accept the truth of his addiction. Even when you think your loved one has "hit bottom"--even countless times, countless "bottoms"--the addiction may prove stronger than the person.
Still, the mom (or dad) who loves an alcoholic (or drug-addicted) child of any age will not give up on that child. She/He won't give up trying to help...trying to get that child to seek professional help.
Wish I had a magic answer, but, sadly, I don't. The person who suggested Al-Anon for the parent has a good point. Al-Anon exists to help the parent, spouse or other loved one of an alcoholic do what is best for herself/himself and break the cycle of codependence. It really requires a distancing of yourself from your addicted child, which not every parent is able to do--even if she or he can see the value in such an action. Trying an Al-Anon meeting or two may help to determine if it's the right choice.