If you’ve done all the standard stuff, clapping out beats, playing drums, etc and it’s not clicking. The best tool I’ve ever found is dance. Try to get him to learn how to move his body in time to music. Doesn’t matter what he’s doing or what kind of dance. Rhythm is something you feel.

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:

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.

Metronome diet.

I play professionally and still use the metronome every single day. I tell students if I can, they can too. Some may see it as a crutch that prevents one from developing an internal pulse. I disagree; but maybe it’s a fake it ‘til you make it circumstance.

If you set the metronome to accent the downbeat, it is difficult to allow yourself the luxury of stopping and restarting on errors.

The most frequent issue though is starting the learning process faster than you can consciously process the music. When a student asks me about the correct tempo, my answer is always the same: it’s the tempo of perfection. If you play a piece no faster than you can play it perfectly, most things take care of themselves.

> it’s the tempo of perfection

I believed in this as well for many years, but I've taken some lessons with famous bluegrass guitarists and all three of them said that you can't really play fast (which in bluegrass is "blistering") unless your body has a feeling for what it's like to be moving quickly at speed without tensing up. All three recommended occasionally taking a swing a tempo that was too fast just to get the feel there.

I've ended up on the following practicing diet, all to metronome:

CYCLE THROUGH:

- 1 rep Slow, No Mistakes (you can think really deliberately about what comes next)

- 1 rep Medium, 1 Mistake Max (you're using mostly muscle memory)

- 1 rep Fast, 3-5 Mistakes Max (focusing on how your body feels moving this quickly without tensing)

Then at the end of the practice session I'll take one or two (or admittedly five or six) inappropriately fast just for fun. I've been informed that this is possibly undoing some of the work I did in the practice session, but I'm playing music to enjoy myself, not to get as good as possible as quickly as possible.

I also do run-throughs at speed to make sure my fingerings are sufficient; at slower speeds everything works

Also I'd have them try setting the metronome to half speed so it counts the 2 and the 4. It forces you to internally feel the downbeat. Plus it's nice to switch things up during your practice session.

double speed?

Half. For instance, if the tune is at 100, set the metronome at 50. And that’s your two and four. The one and the three are in the silence/your body. It ends up at the same tempo.

Right. Not enough coffee when I commented

Not sure how old your son is, but this is what I'm doing with my 5 year old with the same issue. Find some good old-school funk from the late '70s / early '80s with a four-on-the-floor type of drum beat. Zapp, Cameo, Gap Band, or something like that. Get your son to count along "1-2-3-4" with each of the beats and clap his hands on "2" and "4". Once he's reliable at doing that when you don't start it for him, then get him to stomp his foot on "1" and "3", or just "1". Once he's got that down, there are all sorts of things you can do. He can copy rhythms you clap, or if he's good with notation, he can practice writing down a rhythm you clap.

Alternatively, you could always get him drum lessons. This is the meat and potatoes of what a good drum teacher would cover.

Play with him together. Ask him to play a very easy bass line while you are playing / improvising a simple melody, and then change. Do no stop when he stops. Keep the time rigorously and request that from him. My teachers lesson always starts with such a simple improvisation together. It is fun and he will learn a lot.

For improving rhythm I really like this book by Uwe Kropinski and highly recommend it:

https://www.kropinski.com/english/timing-problems-getting-in...