Look, a few different scenarios may be true:

1. He was jacking off to a children’s show that isn’t meant to be porn. Consequence: look into his browsing history and see what he’s been watching.
2. He was bored and just started jacking off while watching his show.
3. He’s using his private time in ways you’re uncomfortable with and need to have a frank conversation about boundaries.

All or none of these may be true at once.

I’d talk to him about it and say you’d like to see his browsing history because what you saw was uncomfortable for you, it was an accident that you walked in; and because of what you saw, you feel like looking through his browsing history on both his phone, computer, and other devices will give you some peace of mind.

Assuming nothing super sketch is in his browsing history, lay down some boundaries regarding his “alone time.”

Are you ok with him watching porn in general? If not, voice your concern about it.

Are you ok with him jacking off when he says he’s up to other stuff? If not, voice your concern about it.

There are lots of things that might be going on, but if you’re concerned he’s watching kiddie porn, you need to find out.