@Thomas B - Don't chalk it up to age as you said. Explain it to him. When it comes to rhythm, maybe use dance as an example. How would someone be able to dance to his playing if he doesn't hold the notes values out completely or rush the piece. Concerning dynamics, tell him that dynamics are like colors, that the colors make the music (painting) more beautiful. Soft piano parts make the stronger, contrasting, more exciting forte climaxes more interesting. If you only play one dynamic all the time, there isn't much of a climax. Soft playing in itself can be very exciting. Have him play the whole piece as softly as humanly possible without changing the dynamics, perhaps with a metronome, then play the whole thing at the same speed as loudly as he can. Most people want to play faster when they play loudly and slower when they play softly. One must feel the rhythm and fight that urge. Afterwards, have him play the piece with exaggerated dynamics. A little rule of thumb when you are not feeling the dynamics automatically is to play crescendos on rising passages and diminuendos on passages descending.
Obviously this is just a way to teach the beginnings of dynamics. One plays stronger on the more beautiful building sections of a piece which is more obvious to some than to others. Have him talk to you about where he thinks the piece should climax to see if he already feels it.
Good luck, and keep up the great work!!!