Try some rhythmic movement stuff - something completely divorced from playing an instrument. Walking around the band hall to music. Breath in and out to music. Raising their arms alternately to music. Stacking cups to music. Whatever. Gradually move to more refined motor skills like opening and closing their hands in time, patting their hands on their leg, you get the idea.
Our brain, regardless of what impairments there may be, is built to keep time (walking, heartbeat, etc.). You just have to unlock the break between their brain and body.