Look man, teenagers masturbate all the time. Guys hammer on it, girls rub it out, it happens. It's not a huge deal. Shit almost everyone has a story about being caught or almost caught by their parents from their high school years.

And you're not the first person to do something stupid because you're horny. Ultimately, he either didn't know or has chosen to ignore it since he hasn't acted differently around you. Don't like do it again, but it's not the end of the world.