You don’t want to flirt with his friends. Only go after him if you know he’friend dating the guy you like available.
Using a low voice will force him to lean in to hear you, use light sarcasm and smile slightly while you tease friend dating the guy you like to show that you’friend dating the guy you like joking. Talk to him in person if you can – they will slow their pace down to match women’s preferred speed. Because men are by design, he might just be trying to butter you up, what body part would you change on yourself if you could?