When doing kiruv whatever you do needs to be done with devotion and love. Getting kids to make them to start doing a mitzvah is one thing. Getting kids to look forward and feel honored joyfully to do mitzvohs is a total different approach. Associate the mitzvah only with positive incentives and introduce it in very special way. Every second he wears the tzitzis or whatever mitzvas he gets schar.Its a good idea to elaborate on that on the childs level of understanding. A nice example of enforcing it positively is if you can get a tzitzis w pictures on it and a kapl with his name and a picture and keep it in cheder and the first thing in the morning very excitedly you can put it on for him and give a little nosh or prize. To instill ahavas hatorah and mitzvos is the utmost importance. If he finds it too pressuring you can start only with 1 hour a day.

Thank you for give me a zechus for trying to assist you with such a beautiful mitzvah. I hope it will be of help.