The ISTP is led by Introverted Thinking which is a form of Introverted Judgment. This means that his decision making must derive from within. In order to be motivated, he must discover reasons on his own endeavor deem his task important. It is not relevant whether or not he will be rewarded or punished for doing so, whether or not others will be pleased, etc. As those factors are external to his inner mindset in relation to the task. He must find the task in itself worthwhile, until he does, at best he will drag his feet at it. If you say that he will be punished for not performing, he will make some effort to perform as he obviously does not want to be punished, but he will not have the motivation to carry through as the task in itself leaves him uninspired.
External exhortations are anathema to Introverted Judgment. The more you force him to go by an external standard the more resistance you will get. This isnt necessarily because he sees something wrong with the particular stipulation, but because he sees something wrong with the very idea of being externally, as opposed to internally guided. He will pull inwards just so he can have an opportunity to make his own decisions. Even if it is obvious that the external decision is very fitting. Unless the decision came from within, to the ISTP, it is simply useless. It is highly likely that when you stop fussing over him spending hours on the computer and the TV, he will invest much less time in those activities.
Do not try to force your views and values onto him. If your ISTP son is to deem his schoolwork important, he must arrive at such conclusion only on his own thinking. The best you can do is simply offer ideas for him to analyze. Inform, but do not direct. Any attempt to inculcate or force your will upon him will necessarily be met with recalcitrance for the aforementioned reasons.
I'd say let him be for now, ease off the pressure. There you will at least stand some kind of a chance of him finding the motivation to carry on with his work. Intervene only if he puts himself by way of grave danger. It is paramount that he be granted as much autonomy as possible. ITPs would much rather have life teach them as opposed to be spoon fed, he will thank you for this when he grows older.