Injury

My cats brother who lives next door has a hole in his shoulder and looks quite deep and is very clean cut. its not bleeding but looks like theres a clear liquid over it. The owners are on holiday and don't know about this should i take him to the vet or wait until they get back on monday.

he doesn't seem to be in any pain and is behaving normally.

Always err on the side of caution. Take him to the vet. Cats are too stoic for their own good, and often mask how ill they are or how much pain they feel. Any wound, especially such a deep one as you're describing, needs to be examined and cleaned.
You don't want to run the risk of infection, so don't wait.

Yes I think you should take your neighbors pet to the vet.....cause to me it sounds serious

At this point, a quick cleaning and some antibiotics will help him heal. If any infection is allowed to get started, it will be harder to clear up. And maybe he was shot with a bb gun, meaning the pellet may need to be removed. A vet visit, for sure!

The "clear liquid" is probably pus, which is a sign there's already an infection in it, which requires antibiotics from a vet. And Beckiboo made a good point--such a wound might be from a BB gun, which a vet should be able to tell.