I don't know how to make this thought coherent, so bear with me people, but. I think ritsu calling shou "suzuki" instead of "suzuki-kun" says a lot about their relationship. ritsu is extremely consistent and polite with his use of honorifics, even with people he doesn't like that much (reigen is always reigen-san, for example)

but shou is always "suzuki," he never gets his proper honorific. this might be a stretch, but I think it fits into what I've thought about their relationship in the past, which is that shou gives ritsu the opportunity to drop the perfect kid act. him using everyone's polite honorifics might be good etiquette, but it also might be him creating distance between himself and others while wearing a polite mask. with shou, he doesn't need to bother with that. they met by beating the hell out of each other for fuck's sake, etiquette is out the window here

but while him dropping honorifics might be rude in other cases (the "NO HONORIFIC??" bit with hoshino in that one omake comes to mind) I don't think it is here. I'd forgotten until doing this reread, but ritsu refers to shou as his friend really easily. like, they've known each other for maybe a full day, and they were enemies before that, but when looking up at the cultural tower, he tells mob "my friend is up there!"

so I think him just calling him suzuki is actually a sign of him being close with shou. no bullshit, no using honorifics to maintain a polite image or to intimidate or to impress. he's just being direct with him, because shou isn't someone he has to put up a front around