My rat has been lethargic for almost 3 days, help please (More info in post)

Me and my girlfriend recently got two rats, Keith and Oscar (brothers from the same litter), they are around two months old and been very active. But recently one of them (Oscar) suddendly got increasiling lethargic; he is also breathing faster and just now I picked him up to check on him and I felt like he's a bit colder than Keith

We tought that he was stressed at first, but he just got worse as time went by, and his brother is doing alright and acting normally so we concluded that he must be sick

We already made a appointement with the vet but we will only go there monday (there aren't any emergency vets who do treat rats in our town), but I'm worried if he will hold on until there as it looks like he's getting slightly worse. He's still eating and drinking a lot of water, but he can't manage to play with his brother and been sleeping all day

He does get more active when I pick him, but in his cage he will just sleep

Is there anything that I could do while we wait until monday?

(Also sorry if anything sounds confusing, english isn't my first language)