If your child has difficulty, stress the beat by clapping and stamping. If your child has a learning difficulty, these points will help him to link what he does with what he hears, what he sees, what he says and what he understands. Fine motor control develops gradually. You may be surprised by your child's competence and creativity with Lego and yet his difficulty using a pencil. Encourage a good balanced grip with crayons and pencils.
Bad habits are difficult to correct if they have been practised from the start.
Teach large arm movements initially, setting up a rhythm. See that the whole body is balanced either when sitting or when standing. If he finds these hard, check that he looks to see where he is going next rather than letting him jump off into space. Similarly practise drawing paths through mazes etc. Activities encouraging gripping and fine manipulation are essential.