I'm really unsure about this one. He's an introverted feminist; we're not dealing with someone who would actually find it funny to be complimented non-jokingly in a lewd way. I know that "You look beautiful tonight" is considered an appropriate compliment, and "10/10 buns, whit-whoo!" is not, but this is somewhere in between. I do genuinely like the trousers (an unusual variation of a specific work style), and they suit him. Would I be better off telling him that they "suit" him, rather than "look good on" him? I'd like him to know that I'm interested, but I don't want him to think he should put his bum away because I'm staring. (I absolutely was not, although in the situation where I noticed, I could've easily got away with it.)
We are friends, by the way. And yes, I know IABU to give it so much thought.