Is he still eating and drinking water? If so, he'll probably be fine til Monday. If he stops eating, and especially if he stops drinking, take him to an emergency vet immediately.In the meantime, keep him warm and calm and happy. And make sure he's well hydrated. If you have a bottle with measurements and only one rat in the cage,keep an eye on how much water is going down. If he doesn't seem to be drinking enough, I like to dip my finger in water and let my ratties lick it off, they always seem more excited about water that way.If he seems like he's having trouble breathing, steam can be great for rats clearing out mucus just like it is for humans. I let my boys run around the bathroom while I have a shower (make sure there are no nasty cleaners/chemicals they can get into first!). You can also boil water in the kettle, but keep them pretty far away from the flow of steam as you don't want to burn their lungs. Also, how old is Gandalf? Very young or quite old rats might need to see a vet sooner. And has he had a respiratory infection before? Also, if you have other rats, it might be a good idea to take them along to the vet with you and get some medicine for them too, I often find if one of my rats has respiratory symptoms, the other one gets them soon after.Hope that helps!