You can't train only one hand as per your preference, allow the kid to use whichever hand he Is fine with it wil gradually get better in precision,if you go on insisting using both hands there might be undue pressure and then it will complicate things
Be patient nothing to worry as such
Next Steps
continue putting small things at desk especially colorful items and indulge the kid in drawing and sketching
Health Tips
do not worry just give some time