It sounds like he should be seen by the vet again. Open mouth breathing is always serious.
Did the steamy shower help him be able to breath without having his mouth open?
Medications for respiratory problems can take up to 3-5 days before you start to see results. If they're not working by the 5th day, your vet should prescribe something else.
I'm wondering if it would be good for him to have an Xray as well?
I'm not sure if Metacam is a good idea to use for treating respiratory disease---I could be wrong. In case it's helpful, here's information on the uses and contraindications for using Meloxicam:
http://ratguide.com/meds/nsaids/meloxicam.php
I'm glad to hear he's still eating and playing. It does sound like a recheck would be a great idea at this point.