Pianoteq is amazing with a good controller like a big Kawai VPC1 or the fanciest Fatar action in the Studiologic "GT" models. It is very responsive. I've been using it for over a decade and the sound keeps improving.

The combination of pianoteq and a sample based piano is pretty nice too, though tough to do on a Pi.

Good speakers improve the experience because you get your room resonance etc.

The coolest thing - you can change temperament. So if you are playing music from before equal temperament, you can hear what different keys used to sound like! Very interesting especially with Bach.

I agree with your son, there is nothing like a real piano. There are interesting attempts at combining the digital and mechanical with soundboard transducers from Kawai and Yamaha, I haven't used them but I would like to.