Is it blackmail or is it giving them an opportunity to see one reason why they should pay him what they already owe?
For context, try to imagine if the company had offered him a "legal agreement" where he was payed a few extra weeks' salary and in exchange agreed not to discuss anything.
Asking for what they owe you is fine, even asking for more is acceptable. It is the threat of an unrelated negative that is blackmail. And that the quickest way to make people unhelpful and if you leave a trace of it, its going to ruin your chances in court.
So no upside just downsides, so don't do that part of "else I am telling the world".
For context, try to imagine if the company had offered him a "legal agreement" where he was payed a few extra weeks' salary and in exchange agreed not to discuss anything.