Hello Louise, I am really sorry this has been happening because overcoming any addiction is certainly not easy, alcohol is a legally accepted material but this doesn't make it any easy than any illegal substance to stop, only that there are places where alcoholics can go without the harm people relate these drugs with.
The greatest problem is that any addiction can and will destroy relationships, families and marriages in so many different ways that we never thought were imaginable.
The main thrust of this is to only hope the person decides to stop by themselves but in many cases, this isn't possible, only because people need to keep using it to carry on with what satisfies them and helps them through the day, as addiction is taken daily and not looked at what will happen in four days time.
If you could look at this 'https://www.healthdirect.gov.au, ICE (Crystal Meth) Healthdirect' may provide some answers.
Finding ways to try and make him rationalise when he's using it can be impossible because what's said is not true, although he believes it's happening and can sway his mind away from the truth and makes your chore to help him, not imaginable.
I have known people to use this drug but were, fortunately, able to stop, although they used it only once or twice but someone using it on a daily basis needs to find the money to purchase it.
It's a difficult position you're in and sometimes, walking away is the only benefit that will help you.