I can't imagine he'd still be at school, going on dates with Hydra, or any of it, if anyone in MLE had any idea.

As long as you're careful not to tip your hand too far, Sirius -- MLE certainly knows you're alive, even if they're not admitting it. That picture that ran in the newspaper was you, and they're perfectly aware they never recovered your journal. If you wrote another public post it would hardly tell them anything they didn't know already.

At any rate, discourage him from playing at Secret Agent if you can; he's far too young, and naive, to run any risks.