that's just plain silly. it's not 'clear facts' you've just told us one more piece of information.
A clear fact is that if you punch someone with a knife, drug-fuelled or otherwise you are much more likely to be stabbed.
Apart from the air-rifle the man in the OP did the right thing, holding his ground, inside his boat.
What are you suggesting? That like Gareth's fantasy you chase them down the towpath, maybe land one punch and get stabbed in return? or "throw them in the canal" and end up in a cell?
That's just plain daft.
And your advice to this man is 'he should have engaged them in violence'? even with a time machine it's not hugely useful to a guy on painkillers who requires a carer.
The advice is obvious, even without Facebook, he needs a lawyer.
A further piece of advice would be "don't admit to a firearms offence on Facebook" but I expect his lawyer will tell him this.