Would he listen to his mother if he listened to her before about the shares? Even if it is just a chat to remind him in the scariest possible way that shares go down as well as up so he mustn't count this money as accessible savings any more...
Also, could you set up a savings account for the Canada ski holiday? Assume it's for all of you, you can drop out later but it will give him a bigger target to save for... Then, daily to start, weekly once he is doing better, work out how much needs to go in to cover holiday, spending money, insurance, parking, ski hire, ski nursery, ski clothes, parking costs, food put there, everything you can think it will cost in total and be generous.
Say if he wants to go skiing he needs to pay £xx into the account every day and get him to do it with the money he would normally use for his coffees and excess food etc. Keep 2 charts on the go - think blue Peter totalizer, one for how much he has saved toward ski-ing and how much has been spent on extra food and coffees.
Make sure you mark it in pencil as I bet he will dig into the savings at least to start with. But hopefully he will soon get to see the way the two totals grow - or don't. If he struggles with numbers as an abstract concept then maybe seeing bars getting bigger and bigger will help him to get a better grasp of what's going on.
Depending on how he does and how he is regarding his spending, I would also be very tempted to wait for him to make his target almost and turn around and stick a chunk into a savings account that he can't easily access to replace your family savings. And then make him save up again... Will he be upset, hurt, angry and so on? Yes. But you are only doing to him what he has done to you. And you are doing it to reinstate savings, you're not frittering it away. You know your dh best and will know if you can do this or if you need to factor in a 'top up savings' element into the weekly saving amount, so that for every £1 saved, half or whatever goes back into savings.
And when you sit down to go through the spreadsheet, get him to tell you how much he thinks he spends on what each day/week/month/year. Then you'll get to see how wrong his misconceptions are before you start and will know where to concentrate. And, again if he doesn't like numbers, use a pile of Lego blocks or monopoly money or casino chips or home made money and props to help make it real.
Also look on the moneysavingexpert.com site to see what resources there could help. I think they have a booklet aimed at teenagers or students to act as a intro guide to managing your finances and being financially aware. Might be something you could sit down and go through with him that might help to explain stuff to him.
Good luck and I hope you manage to get him to take you into consideration as well as himself, be it financial or getting to go ona holiday of your choice sometimes!