Could be an upper respiratory infection. In the meantime, keep him away from the others. URI's are very infectious (and common)

Most of the time, they run through their system on their own (like we do with a cold) but its always best to see a vet and see if they recommmend meds. Waiting til Wed won't be the worst thing in the world. Just keep an eye on him and watch that the sneezing doesn't get worse. Make sure he's still eating and drinking.