It really depends what his temperament is like. If he's like I was when I was young and is really stubborn, resistant to practicing stuff that matters in favor of stuff that's fun, and doesn't listen to your advice, then you'll probably need to find a local talented adult musician that he latches onto and hero-worships to teach him rhythm. I don't know what they'd do so I can't predict anything there.
If he listens to you really well and wants to collaborate:
1) You can play a lot of rhythm games all the time. Here are a few starter videos with source material to improvise rhythm games off of:
As a game for when you're waiting around with him in line or in the car or whatever, I also recommend challenging him to try to keep time differently with different limbs--can he tap quarters with left foot, eighths with left hand, halfs with his right foot, and the melody with his right hand? Can he cycle these responsibilities randomly on his limbs?
You can also play songs and challenge him to identify the time signature and count along to it, or sing the songs and clap along at the same time.
2) Listen to a lot of music together, and dance/snap/tap/head nod to the music. Really rhythmic music is good in this case--Irish, Bluegrass, Gospel, R&B, and Motown are typically the most exciting and danceable.
3) Play "call and response" improv games. Sit at the piano with him, play a jazz backing track, and play and sing him simple rhythmic phrases to play back by ear alone. "Quarter Quarter Quarter Dotted-eigth-sixteenth!" etc.
4) Get him drum lessons.
5) If he's totally rhythmically deficient, let him have piano as just a fun random hobby and teach him to fight instead ;)
The most important thing to remember is that a good sense of rhythm can take 5-10 years to really develop. It's more important to attack the same thing every day until it's really mastered than it is to press forward for unearned progress.
If he listens to you really well and wants to collaborate:
1) You can play a lot of rhythm games all the time. Here are a few starter videos with source material to improvise rhythm games off of:
Rhythm Yardstick: https://www.youtube.com/watch?v=8Sw_trDFJw8
Counting Lesson: https://www.youtube.com/watch?v=dALpbzL7xqo
Counting to Seven: https://www.youtube.com/watch?v=Njy7pMVKJ8Q
As a game for when you're waiting around with him in line or in the car or whatever, I also recommend challenging him to try to keep time differently with different limbs--can he tap quarters with left foot, eighths with left hand, halfs with his right foot, and the melody with his right hand? Can he cycle these responsibilities randomly on his limbs?
You can also play songs and challenge him to identify the time signature and count along to it, or sing the songs and clap along at the same time.
2) Listen to a lot of music together, and dance/snap/tap/head nod to the music. Really rhythmic music is good in this case--Irish, Bluegrass, Gospel, R&B, and Motown are typically the most exciting and danceable.
3) Play "call and response" improv games. Sit at the piano with him, play a jazz backing track, and play and sing him simple rhythmic phrases to play back by ear alone. "Quarter Quarter Quarter Dotted-eigth-sixteenth!" etc.
4) Get him drum lessons.
5) If he's totally rhythmically deficient, let him have piano as just a fun random hobby and teach him to fight instead ;)
The most important thing to remember is that a good sense of rhythm can take 5-10 years to really develop. It's more important to attack the same thing every day until it's really mastered than it is to press forward for unearned progress.