@Corkingbig I said much the same earlier in the thread.
Unfortunately, normal and all as a sober alcoholic is, there are things that they just can't do. That takes years to come to terms with sometimes and sometimes you make mistakes. If you could compare it to someone missing an arm. Yes, there's lots of things you can do, but for example, boxing might now be out of the question.
Your DH put himself into an extremely dangerous situation. 1 Music is a massive trigger for some. 2. His feeling of joy with spending a good time with his son, also a massive trigger for some. 3. Surrounded by people drunk and drinking merrily.
What he could have done however was just had a few with your son still with him, unless your son is fearful around his Dad drinking (sometimes from things the alcoholic does or has done, sometimes from things well-meaning people might have told your son). In such a situation he might not have gotten shitfaced etc., but perhaps he knew he'd face an earbashing when you found out. For whatever reason, he didn't feel he could drink in front of your son, the craving got the better of him and well I'll be the last to cast judgment on that.
I am sure he feels like shit. He behaved appallingly in normal terms.
But he's safe, your son is safe.
An alcoholic has a disability. Can you cope with that? That's the question you've to ask yourself.