Boyfriend walks around shared flat in his pants

Less weird if guys but I think that's a bit weird if you have 2 other girls tbh. Have you asked him about it?

My housemate does it, I think it's fine if you have that sort of nothing-but-platonic relationship that housemates often have

A couple of my male flatmates walk around in their boxers most the time, one of them has a girlfriend and she doesn't mind it as she knows I and the other girl in the house are completely platonic with him and she trusts him. I sometimes walk around the house in my underwear too, it just means he's comfortable with himself and around his flatmates. Nothing wring with that. If it makes you uncomfortable, explain to him why.

It'd be better if he at least wore boxers or put some shorts on. That's acceptable I think

Wait what is he wearing?

pants usually, sometimes tight boxers

Grimy.

Have any of your flatmates ever commented/complained about him doing this?

Right I see, I don't know anyone who weird briefs so that in itself I would find odd just because they're more seen as children's underwear. But obviously his choice in underwear is fine

Even still I myself wouldn't find it weird, but if you find it weird and uncomfortable explain to him WHY.

I wouldn't worry too much about it. It's surprisingly normal during student life to spend half of your time either in pyjamas or underwear.