As a point of clarification, this would be a piano quartet, not a quintet. 🙂
I really like your son's musical ideas here. The minor keys are typically more difficult to navigate, but your son has done a marvelous job constructing phrases and transitions that take us quite convincingly to each section of the piece. I agree with everything @Omicronrg9 says concerning the piano and the pedal string parts. (For example, if I were a cellist, I would be bored just playing staccato quarter notes for the entirety of this piece.)
Here are some things that might need to be changed. First, the viola has no need to use the treble (G) clef in this piece, and I would put everything back in the alto (C) clef. Second, he has the cello part playing fortissimo (ff) in some places while the other instruments are at quieter dynamics. This needs to be revisited, as well, or the cello will sound out of place in an actual performance. Third, the piano's left hand could really use more movement. It looks like it just doubles the cello (or vice versa) with quarter note after quarter note while the right hand gets the actual workout. Not much fun for a pianist. Not to worry, though; all of this is easily addressed with more experience.
Thank you for sharing. Wishing your son the very best!