1) windows 10 tablets start as low as $200, Sure, those will not be able to run VST and DAW software.. But will run Yem and most of the 3rd party Genos apps just fine.
2) You have to plan ahead. Chances are huge he will want to use it for YEM and other 3rd party apps in the future.

Musicsheets pro will run just fine using Androidplay on any Windows tablet.

And by choosing Windows as the base OS on an entry level tablet, it will be much easier if he ever wants to to upgrade to a more powerfull Windows tablet if time arises.

Windows tablets based on atom processors are just as affordable as Android tablets.