Watching TV all the time could affect his health, like it could make his eyesight less, his fat would also increase and these could lead to other diseases.
What you could do is tell him its not good to watch TV programs all the time. When you go for playing you could call him also , so that slowly- slowly he get interested in it. And while playing during the break time you could make him aware of the bad consequences of watching TV all the time.
HOPE IT HELPED..