It depends on your contract. If you agreed that it would work under a certain version of the IOS then that’s all he is required to do.
Look at it this way.
You app works with IOS x.0 and he finishes the work and you sign off.
If he delivers the app fully functional is IOS x.0 he’s fulfilled the contract.
The next day, Apple releases IOS x.5 and your app no longer works….is that the developers fault?
Nope, you would have to pay someone to re-do the app.
If you didn’t anticipate Apple releasing a new IOS and didn’t build it into your agreement…that’s your problem.