This can affect him in many ways which include:-
1) He may face problems related to eye sight.
2) He may loose his concentration in studies.
3) If you watch TV in access then it may affect your sleeping time which can lead to depression or sleeping disorders.
He can be encouraged to take up more productive activities:-
1. You can aware him about the consequences which can lead to excessive watching of TV.
2. Ask him to balance between studies and TV. Ask him just to spare maximum 1.5 hours on TV not more than it.