I'm a nearly 40 year old father of a teenage boy. I walked in at night to check on him, and he was masturbating. I watched until he ejaculated. He wasn't using porn. I quietly exited, and I'm positive he doesn't know I saw him. I do not know what to say to him. When I was 14, I had my first ejaculation by surprise. I had a wet dream, but didn't know what they were. I continued to masturbate daily until my father caught me. I wasn't using porn. I wasn't even thinking lustful thoughts, but my dad said that I should not masturbate because it will always lead to horrible sins, such as porn. He said the more I ejaculate, the more I would want more. He said it led him in the wrong path. I stopped masturbating, making a promise to God I wouldn't ever masturbate in my dad's house again. I kept that promise, and haven't done it since. I do not want it to be a block in our relationship. Also, I am always cautious not to reveal or let my son see my penis, but if I tell him I saw his, should I let him see me?
Only God is the determiner of what is sinful. Just because your father said it was wrong, doesn't necessarily make it wrong. It has to be founded on Scripture. "If anyone speaks, let him speak as the oracles of God. If anyone ministers, let him do it as with the ability which God supplies, that in all things God may be glorified through Jesus Christ, to whom belong the glory and the dominion forever and ever. Amen" (I Peter 4:11). See: Is masturbation sinful or not? for a study on this issue.
Watching your son, who wasn't doing anything wrong, during a private moment was wrong. Nakedness is something that should be treated as embarrassing. See: Is seeing other boys in the nude all right? One of the reasons Habakkuk condemned drunkenness was because some used it to see others naked. "Woe to him who gives drink to his neighbor, Pressing him to your bottle, Even to make him drunk, That you may look on his nakedness!" (Habakkuk 2:15). The proper response should have been to note that no porn was being used and then quietly leave. There is zero reason to tell your son you saw him. There is even less reason for you to expose your nakedness to him.
What bothers me about your note is the double standards you portray throughout it. You believed masturbation was wrong because your dad said so, but you didn't tell your son that. You knew it was wrong to expose your nakedness, but you spied on your son's nakedness and are now talking about exposing yourself. Even though the moral standards are off because they are not based on Scripture, they are still not being applied consistently.
Your son does need instruction on what his happening to him and his responsibilities as an adult male. There is material that you can use to study with your son, and perhaps in doing so you will learn more about your own responsibilities. See: Growing Up in the Lord: A Study for Teenage Boys.
After reading your message, I saw that I was completely in the wrong. I only want what's best for my son, but I stepped way over the boundaries. I also realized that I need to respect my son's privacy, as I would want him to respect me. I didn't tell him anything about what happened, and I'm not. I will talk to him about masturbating and the temptations that come with it, but I'm not going to tell him it's wrong.
I really appreciate you for helping me realize my duty as a father. And after thinking about exposing myself, I realize that's way off proportion. I'm going to pray to God to give me wisdom in this matter.