Thanks for the additional information. It is helpful. His current behavior is likely from having the glands expressed. Often if the glands are already giving them some problems, the area will continue to be uncomfortable for a few days after being expressed. Warm compresses on the area seems to provide relief. Since you did take your dog in, they should have examined him before expressing the gland and seen any other obvious issues. Expired metacam is likely not to be effective since it loses its effectiveness over time and thus I wouldn't give it to him. I doubt it will help. The compresses should help. I would expect discomfort to start improving in the next day or so.
Now if he has ingested a non food item, bone, rawhide etc that might be moving through his system, keep an eye on him for vomiting, diarrhea and the passing of a non food item. You might add a little canned pumpkin to his food (not pie filling). It will help keep his feces firm but pliable which will help prevent loose bowel movements that contribute to anal gland issue. It will also prevent hard feces which might also cause a biting action toward the rear.
If this doesn't help and he continues to bite at the area beyond a day or two, then I'd have him reseen by the vet. If a fecal wasn't done, have that done as tapeworms can cause a dog to bite at the rectal area. I would hope they ruled that out as a cause when you had him in, but sometimes things get missed.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer.