Had to name change for this due to information included
DS1's Dad is an alcoholic. He got progressively worse, putting drink before DS, and becoming abusive and violent. He had his chances, we left. Have had many incidents with him, but finally got him reasonably well trained with DS and them spending time together. He wouldn't listen re the drinking, and was drinking excessively when DS was with him, so I reduced his access to the afternoon to limit what he could get up to. Long story short, he had a heart attack which frightened him in to stopping drinking. His health improved along with his relationship with DS. Despite the improvements he had another heart attack, and gave up smoking as well.
Over the last few months, some of his so called friends encourage him to have wine, because it 'didn't count' apparently... this came in a pint glass 
and quickly reached silly amounts. He is now drinking ALL the time to excess. He won't listen, and will probably end up having another heart attack, leaving DS to suffer if the worst happens. I am at my wits end with him. If it wasn't for DS, I wouldn't care, BUT... it's so hard to watch your child deal with this.
Dropped DS off to him yesterday at 5pm, he took him straight to the pub to watch the rugby, and this afternoon spent in the pub drinking and watching the rugby. He got a lift to drop DS off 
He's not even a rugby fan, it's just another excuse to drink in the pub! I really want to slap him at this point in time. It's pointless for DS to spend time with him.
Probably relevant: DS is 13, and these access / drinking issues have been going on for 10 years. Mediation people were useless and let him get away with loads. I don't know if I should get legal advice and go to court to prevent him drinking when has access as I don't want to make any of this harder for DS. DS does want to see him, but not drinking / in pub. Any advice please.