It's the unrivalled king of score writing software and absolutely the dog's bollocks. It's distinctly designed for professional score writing and to deal with all the weird and wonderful instructions issued by the most bizarre composers of modern avant-grade music. But it costs a king's ransom and is probably overkill.
There's plenty out there nowadays that's free or cheap, I'm using muse score for Linux.
Yes - as 'boris' says, it is THE system for today's music.
But, at £80, possibly less if you shop around or you can purchase with the help of school (which can 'lose' VAT) I would hardly consider this entry-level version of Sibelius expensive!
Our DS first had Sibelius twenty years ago, when it was available to run ONLY on Acorn RiscOS computers. The software, complete with computers, was sold to Hollywood film studios, and professional musicians around the world. (The ARM chips evolved from Acorn, went on to become the standard chip for most mobile phones.)
It was created by twin brothers, Jon and Ben Finn, who were studying music at university, but there was at that time no software sophisticated enough to do the things they wanted. So they wrote their own, in Machine Code, for Acorn computers, which was a standard schools' computer in those days.
As it got more popular, it was adapted for PC and Mac, and eventually was bought up by the US Avid company.
Yes, your E323 is certainly compatible with it, as the keyboard has MIDI and is 32 note polyphonic (I just looked it up!) which means it can sound 32 notes simultaneously, so can accommodate complicated orchestral or band scores. You will need special MIDI cables to connect the items.
I do hope DS is learning to play 'properly', and not just using the E323 built-in tutor system (I do not like automated tuition systems, and it is really NOT that difficult to learn to read music and play properly.)
Certainly, Sibelius will have many features and facilities that DS won't use until he is much older, and is writing scores, arranging and playing to a standard approaching that of professional musicians. If he does eventually want to take his learning to that sort of level, then the software won't be a limiting factor. If he ever wants to go BEYOND that level, he will probably be earning enough to buy the £600 professional version for himself!
(MIDI is a complicated computer/music interface, that I have never really got to grips with. It too has been around over twenty years, but of course, has been advancing and changing throughout that time. Computer programming is also a useful offshoot that can be used at university and in the workplace.)
If you need any more clarification or help, let me know.