One thing I've learnt in life is, everyone makes their choices. When people are skint, there's a reason for it. It's because they've spent all their money.
Very few single people living alone don't have any income in the UK and even those in that situation can change it with some form of help, whether that's claiming benefits, getting a job or returning to whatever country they came from and are entitled to help from. So pretty much everyone has an income from somewhere. Or has someone else, eg a parent or partner, paying their living expenses. Very few people can't live on their income. It may be tight to balance their finances and life may seem hard or miserable and their standards of living may be far from what they want, but if they stay out of debt it's usually doable to survive on whatever their income is.
So you say he's cavalier with money, but what makes you think he needs "saving", by you or anyone else? Just because he's not living like you do, it doesn't mean he doesn't have a plan. Maybe his home is social housing and you just don't know that or may be he's been on the waiting list for it for decades and his turn will come up when his landlord sells and he's made homeless. Maybe he detests cleaning and is happy living in an HMO, having only one room to clean and paying less rent than on a flat. At retirement, if all he's got is state pension, he'll get his rent paid by housing benefit. If he's living in social housing or cheap enough private rentals then it'll cover all his rent. That could put him in a better financial position than someone who owns a home but has to pay all the service charges and repairs themselves out of their state pension. Or maybe the reason he's skint is because he's funnelling 50% of his salary into pensions and savings, in preparation for his old age. I knew one person in poor health who happily accepted any debt they could get, on the basis that they expected to be dead long before they reached the stage of not being able to make the minimum repayments. They weren't remotely stressed about the debts and didn't have any kind of guilty conscience about never paying it back either. They worked, they spent and they enjoyed life, not expecting to live into retirement age. You really don't know his true circumstances. You only know what he tells/shows you and that may only be part of the truth.
I often tell people I "can't afford" something when it's not strictly true, it's just that I want to spend my money on something else or as a polite let down rather than telling them I don't want to do xyz at all/with them. He may be perfectly happy with his life and the way he's living it. What you see as money mismanagement may be be a conscious choice to prioritise fun in the present over saving for a future that isn't guaranteed to exist. And if he is just plain feckless and miserable with it, well, that's still not your problem!