Oh dear!!! I feel that I have stepped back in time by very many decades as this is exactly what happened with my older brother, back in the 1950's. It ended so badly with he and my Dad having a dreadful row - about never giving any rent money, etc. and then a fight, with him knocking down my Dad. I can remember my Mum screaming at him to GET OUT, GET OUT. Then as he was walking down the road, giving me his Post Office Savings Book and telling me to run after him to give him that.
Was about two years before we saw him again!!!
If this man is unemployed, then he should be claiming benefits and he should be handing over a large proportion of these towards the cost of his keep. If his parents really do not need this money, they should still insist he takes it and can always (secretly) save it for him for the future.
I did this with my eldest daughter over thirty years ago for a year. She thought I was the worst of parents at the time, I had to use some of it, but saved fifty percent, gave her that lump sum when she did start to sort herself out and moved into her own place. She now has a wonderful career, beautiful home and says it was one of the best lessons she ever learned, that nothing comes free. She has always, since then, handled her money so well, even during times of serious financial hardship.
This man could be depressed - he should be persuaded to seek advice from GP. Both parents need to sit him down and talk to him together (it should not be a Dad or Mum talk). See what he thinks is his future - a mechanic should be able to get a job even at present. Perhaps, if he is in or heading for depression, it should be a part-time job to start with.
benefits money needs to be handed over, and rules laid down as to his behavior and attitudes whilst living in someone elses home.
Nothing is easy when dealing with out beloved AC. But allowing him to get away with this sort of behavior is exactly the same as allowing toddlers and small children to run riot and not correct them. In the long run, more cruel than kind.