We have found out that our oldest child (13 years old) was watching filthy pornographic videos on the internet
Trying to block a child from viewing Internet porn will NEVER work. The blocking software is utterly useless, as it allows so much porn to still be accessed. If you take away his Internet service altogether (buying him a flip phone, for example), then he can't do legitimate homework. Plus, he will have boys at school who will show him inappropriate web sites (as a high school teacher, I see this a lot--we can't do anything--public school after all). Thus the ONLY thing that works is to build up a spiritually based internal drive in him to build a fire resistant Christian personality that fortifies him to the point that he can resist this depravity (we all know the extremely damaging effects of porn addiction, to single and to married individuals). Here are some general Biblical guidelines that I have collated from different sources that will help:
Step 1: Guard Your Eyes
Feeding our eyes on sensual images will only fuel improper desires. Confirming the link between seeing and desiring, Jesus warned: “Everyone who keeps on looking at a woman so as to have a passion for her has already committed adultery with her in his heart.” Using vivid hyperbole, he urged: “If, now, your right eye is making you stumble, tear it out and throw it away from you.” (Matthew 5:28, 29) The point? To resist temptation, we must act decisively and not feast our eyes on provocative images.
Try this: If you glimpse erotic imagery, quickly avert your eyes. Imitate the Bible writer who prayed: “Turn my eyes away from looking at what is worthless.”—Psalm 119:37.
Step 2: Guard Your Thoughts
Because we are all imperfect, at times we may struggle with wrong desires. The Bible states: “Each one is tried by being drawn out and enticed by his own desire. Then the desire, when it has become fertile, gives birth to sin.” (James 1:14, 15) How can you avoid being caught in such a downward spiral? When you experience wrong desires, remember that you can choose how to respond to them. Fight those desires. Tear them from your mind. Refuse to dwell on immoral fantasies.
Try this: When you are plagued with immoral thoughts, immediately stop and pray. Fight wrong thoughts by filling your mind with “whatever things are true, whatever things are of serious concern, whatever things are righteous, whatever things are chaste, whatever things are lovable, whatever things are well-spoken-of, whatever things are virtuous, and whatever things are praiseworthy.”—Philippians 4:8.
Step 3: Guard Your Steps
When desire, temptation, and opportunity converge, trouble can easily follow. (Proverbs 7:6-23) How can you avoid becoming a victim?
The Bible wisely counsels: “The shrewd one sees the danger and conceals himself, but the inexperienced keep right on going and suffer the consequences.” (Proverbs 22:3) So guard your steps. Anticipate situations that could trigger problems, and avoid them. (Proverbs 7:25)
Try this: Honestly evaluate your weaknesses, and plan ahead to avoid situations that could expose you to temptation.—Matthew 6:13.
DO NOT GIVE UP!
What if, despite your best efforts, you weaken and give in to temptation? Do not lose heart and quit trying. The Bible says: “The righteous one may fall seven times, and he will get up again.” (Proverbs 24:16) Yes, our heavenly Father encourages us to “get up.” Will you accept his loving help? Then never tire of turning to him in prayer. Build your faith by studying his Word. Strengthen your resolve by attending religious meetings. Take heart from God’s promise: “I will fortify you, yes, I will help you.”—Isaiah 41:10.
) and I don't ever feel guilty if I spend the day out hiking but I do when I play a video game all day. 
