1) Its would be reasonable and just to pay the developer for the improvements made in new versions except for his very clear promise never to charge for upgrades, a contract he made that is valid under Italian, EU, US, and international law generaly.
The develper pleads he "made a mistake" making that offer. That is not a defence. One can understand his problem, but not excuse his actions He should not expect others to pay for his mistakes, mistakes he alone is responsible for.
2) Speaking of mistakes, paying developers for correcting bugs after the initial purchase payment, is a commonly accepted but absurd practice that is tolerated nowhere except in the software industry. In this case however the same promise exists never to charge for such corrections.
3) The develper should reconsider his decision and apologize, in order to restore whatever good will and reputation he has earned thus far with his product.