What am I supposed to do?

It is hard because you have all slipped into this pattern of behaving and you know the longer it goes on the worse it will get. I don't want to sound harsh but I look after an 11 year old and we are teaching him to budget his pocket money so when he gets older he will have that skill. If he wants to buy something we discuss how much he could put aside each week until he will have enough. We could easily buy it for him but then he would not learn. I would start by charging more for him to live with you, enough to cover all of his costs. I would make it clear he needs to put money back each time he is paid to pay for fuel later in month.as there will be no more loans. I would ask that previous loans be repaid and I most definitely would not allow smoking in my house. I would make it crystal clear if he smoked one more time in the house he would have to leave. It is called tough love, training him to cope on his own because you won't be around forever and the older he gets the harder it will be to change. Also giving money to this son without it being repaid is unfair to your other son who you can be proud of.

I think you need to sort out some grounds rules and stick to them. If you don't like him smoking in the house, make him go outside and don't turn a blind eye.

If he wants to drink and party like a teenager that's fine but not if he can't afford it and has to "borrow" from you. Did he still get paid when he phoned in "sick"?

It's time he took some responsibility for his actions and grew up

Time for you to have the talk. Tell him you are not in a position to support him, he has to stand on his own two feet. Tell him you will no longer lend him any money so he has to learn to budget. You don't say if he is paying you anything for his keep, if not, then he has to start. What does your partner think of all this?

So. A forty year old who dosses at his mother's house, smokes although it is forbidden, drinks his money away to the extent he can't afford to go to work, sponges off his mother, and when she won't sub him, calls in sick.

You feel guilty, and so you should.

By tolerating this behaviour you have encouraged him to develop the persona of a lovable rogue, whereas in reality he is a feckless spendthrift, a scrounger and probably a drunk.
The answer is quite simple.
Boot him out, as presumably his wife did.

I don't see why a 40 year old without any dependants can't live a life of Riley, (lucky sod!)
You shouldn't be financing him or running around picking up after him though.

Believe me chinesecrested he won’t be sleeping under any bridges, he’ll get his finger out and act like a man, about time too.
You treat him like a child and surprise! He’s continuing to act like one.

Time to let him stand on his two 40 yr old feet. Trust me you’ll be doing you all a favour.

I think you need to sit down with him and have a frank discussion. He is a middle aged man and needs to act like an adult, at the moment he is living the life of a teenager. I presume that he is not paying any board and hope that you are not also doing his washing and cooking etc ? in all honestly there is a difference between help and support and enabling,in this case you are enabling him to lead a life without any responsibility.
It is worrying if without any dependents he is running out of money and borrowing from you.
As you say you may not always be around to help and what then? Better he starts to stand on his own feet now.
I understand you don’t want to see him homeless, but can’t you help him draw up a plan with a budget to allow him to save up to move into his own place?
There is a useful budgeting tool on the money service web site if that would help? If he is on a low income get him to check any benefit help he might be able to receive, so he can afford to live independently. Turn2us has a good benefit calculator to check

This sounds appalling, Chinesecrested! I can't help but think you're perpetuating his irresponsibility as he is still living like a feckless teenager! My advice would be to set a deadline with plenty of notice, by which he has to have moved out and live on his earnings.