Hi, a child can react like your son for different reasons. Kindergarten is a big change for these little guys. He may feel overwhelmed by his enviroment, anxious, etc. He may indeed be bored. He may have some underlying areas that make the work difficult for him. In the US, kids are evaluated heavily (not just by special order, but all kids are generally evaluated by a speech person, an ot and a psychologist---- they try very hard to detect a child that might need early intervention so they can be successful in school.) I'm not sure where you live. But working with the teacher and director of the school is your best hope. They stack him up compared to his peers and can give you insight as to where he should be. And they can help get them there. They will expect you to work with him at home.
TV is deceiving. Kids who can't focas (and ADHD child for example, zone out watching tv.) But that is something I would try to limit. I wouldn't necessarily say "be good" at school for tv because he may be trying his best but have an underlying issue for why things aren't going well. What I would do is have a "homework" time every day (all 7 days of the week)---- and make it a short period. First do some physical activity beforehand--- run and jump and climb. then sit down to work for 10 to 15 minutes. Increase the time as he gets used to the habit. By having this sit down time, he then can earn a half hour of tv. Something like that.
My son has a mild delay called sensory integration dysfunction--- we've been doing this homework thing for a little while now just to get him used to doing his work. They started it with him when he was 4 (he's now 5). anyway, stay away from words like bad and good to describe him. It hurts kids self confidence which I'm guessing he is starting to lack anyway with trouble at school. Good luck.
If he really enjoys tv I would tell him that everyday he makes it through school and his teacher says he was good, he can watch his favorite show. My daughter also had an issue like this and it put her behind in class. Her teachers thought she needed to attend a class for children who learn slower so they had her do an IQ test and her test scores were more than above average. They reccommended that we move her up a grade because she was bored. We did move her up a grade and she is now at the top of her class and follows instruction.