Here's what you do: realize YOUR mistake. You see, YOU interpreted no response as "Hey let's walk in here and traumatically view a family member doing something in the privacy of their own space!" It doesn't matter to me what ANYONE is doing if they're behind a closed door. Closed door is enough for me to know that it's something I either don't want to see or they don't want me to see.
The whole point of knocking is so that they may acknowledge your presence before you enter. If you don't have a response, just assume they're sleeping and leave them alone.
If you talk to him about it, he will be pissed that you came into his room without his knowledge. That's an invasion of personal space, yo.