Asking someone you're casually dating to formal?

[shimshamhotdamn]

Hey all (sorry! I'm new here! Let me know if I screw up!)

I'm a sophomore newly initiated member of my sorority, and I'm still learning conventions, including those for formal, which is traditionally held on the last day of classes here. Less than two weeks away!

I've been semi-seeing a guy (like two meal dates/week, and going to a party or hooking up or something outside of that, plus a fair amount of texting) for a little under a month - we have yet to DTR (and I'm pretty newly out of a serious relationship so I don't really want to) and I'm concerned asking him to formal will have Implications - but also, he's in a frat/involved in our pretty small, insular greek life community and will know that I didn't ask him if I don't, which may also have Implications.

Another complication: I'm definitely doing dinner with my big and twin, both of whom are bringing female friends as dates, and my grandbig, who has graduated and isn't actually going to formal - so bringing a guy means he'd be outnumbered 6 to 1. This guy is probably fine with that, but I feel like that could also be weird.

My roommate has already said she'd be happy to be my "date", so that's an option - but aaah! Is formal more fun with a guy (some people say babysitting gets boring) or without one? For reference, we always do it in a bar/club rather than a house or something. Is asking someone I'm casually dating a bad idea? Will dinner be weird? What is the protocol about asking someone? I'm running low on time and don't even know how to ask him while giving him an out. Thoughts?