Brace yourself for a serious conversation, or a series of them. If he'll agree, rehab is brilliant and some GPs will arrange it. Perhaps your DP could check it out?
I don't think I could have done it without rehab - 6 weeks away from everything and everyone, focussing on nothing but my addiction. But then I've meet loads of sober alcoholics who've succeeded just by going to AA meetings. 90 meetings in 90 days is what's recommended.
Rehab gets you started, but you need meetings afterwards. I don't go to meetings any more but I did 5 a week for a couple of years, and didn't stop going regularly until I'd been sober about 5 years. I'd go back like a shot if I thought I might want a drink.
This is where the support comes in. It'll be tough for you and your DD if your DP has to go to an AA meeting every day rather than coming home. But you smile and say "Go". AA meetings, lots of AA phone calls. It's a lot.
No one has mentioned Al-Anon - for the families of alcoholics. This I'm not so sure about - I went to some meetings myself and found them very patchy - but some people swear by it. You'll find it at www.al-anonuk.org.uk/
Alcoholism is a family disease - it's genetic but it's also learnt. You may find he has relatives with a drink problem, or maybe you do. People brought up by alcoholics often live with alcoholics. And it is also a real illness. You can see it close up in brain scans, and at long distance in epidemiological studies. Your DD needs protecting from learning alcoholic behaviour. You can't do anything about her genetic inheritance but you can watch out for her as she gets older. One of my sons seems to have the tendency, the other not but they both know that me and their grandad are alkies. Sober, but still alkies.