Yamaha DGX Apps?

Hey guys!

I just got a used DGX-650 for my son and so far so good. HUGE upgrade from his previous keyboard and he’s poking around into the synth already.

Trying to stay a step ahead of him, I am wondering if there are certain apps you guys recommend? I check the Yamaha USA website and it is, uh, lacking to say the least.

https://usa.yamaha.com/products/musical_instruments/pianos/portable_grand/dgx-650/apps.html

I get that it’s an older keyboard, but only one even exists in the App Store.

Anyhow, I figured he’d like a sheet music app and maybe something Synthesia-ish if you guys have any favorites / recommendations.

Bonus points for any special cords / connectors I might need.

Thanks!