It is very possible that the stress of the visit, even if he didn't seem to be stressed, has brought on an upper respiratory infection. If he continues sneezing, becomes congested, and/or starts having runny eyes, then you need to be concerned and take him back to the vet. Also, if he stops eating or has a decreased appetite, change in sleeping, playing, and/or litterbox habits, that's also cause for concern. You can try steaming him in the bathroom. Just get the bathroom nice and steamy, then take him in there and close the door and sit with him for about 10-15 mins.
For whatever it's worth, almost everytime I take Callie to the vet, almost 2 weeks later, she gets an URI. The vet knows this, so now all I have to do is call, remind him she was in 2 weeks previously, tell him she's sneezy and snotty, and he'll prescribe medicine for her.