Seeing as they all know the business is closing down, it actually makes it really, really easy. Usually a big part of the etiquette dilemma is how to explain that you are looking for a new job and why, and ask them to be a referee when obviously they don't want you to leave (or if they DO want you to leave then they're probably not the right person to be vouching for you as a referee!) but here it's easy.
Pick a time where they're not too busy with work and it won't be a huge interruption, then just ask:
"Obviously I'm going to need to get a new job in Xmonth, would you be happy to be a referee for me for future employers? I'd really appreciate it." They will most likely say "Yeah of course!" to which he should just check which is the best contact details to list etc. He should also ask if they'd like to be kept updated when he's interviewing for roles so they know about prospective calls. (Personally I don't list referees on my CV, I provide them after interview on request. This gives me time to call my referees, tell them what role I'm interviewing for, and ensure they will be available to take the call. But that's a personal preference. I know companies who will call referees up front prior to interview and without telling a candidate, and I don't want to waste my referees time like that by having to take lots of calls from jobs I won't necessarily get or take).