Dovahkiin, yes, you definitely pass as a prepubescent boy, I wouldn't blink if I saw you in the men's toilets.

I think a small advantage is that most women don't even like the men's toilets. They complain about them being smelly (which compared to the ladies they ARE. But I don't care, lol), and women don't like being in 'men only' space I think. So men REALLY don't expect a woman in there. If they WOULD look and would see someone who's doubtfully male they'd probably shrug and think he's a feminine guy or something.

He should be fine.   I definitely waited until I was on T until I started using the mens room.  I realize, however, that this isn't an option for everyone, and I didn't publicly make the name/pronoun switch until hormones and surgery had been completed (only my gf at the time knew anything about it). The bathroom switch came naturally along with everything else.  I can't even remember fussing about it really, or when the first time even was.

Men are unobservant.  I think the safest time to make the switch is when you have women telling you you're in the wrong room half the time you use the restroom.