From my quick look at the differences not much. I just got curious and listened to sound samples of the 90X because they changed the sound engine. I don't hear much from the demos. Mine does all I want plus it midies seamlessly to my BK7m. A NOS FP90 will save you a good amount of $$$. Amazing how prices have jumped. I see used ones for more than I paid new.
Nice story about your grandson, keep the music alive. And it sounds like he well above the typical 3 chord guitar shredders and playing some good music