Hello, I'm Dr. Deb. Thanks for requesting me; I'll do my best to help you today.
I'm sorry that Smokey is constipated although this is a fairly common problem in many cats as they age.
You do have over the counter options which may help until you can have him seen tomorrow...if he still needs to be seen that it. You could use several of these options together if he's significantly constipated although if you do, his stools may be a little loose for a few days.
1. Hairball laxatives such as Laxatone or Laxaire which are brand names but every pet or grain store will carry these products. I usually suggest 1-2 inches 2-3 times a day.
2. Canned pumpkin (NOT the pie mix) at a dose of 1/4-1/2 tsp per day. You might be surprised how well many cats like the taste of it.
3. Over the counter, human Miralax at a dose of 1/8th-1/4th tsp daily which can be increased until soft stools are produced.
4. Rawbran flakes (wheat bran, not cereal) in canned food 1/4-1/2 tsp daily.
5. Pediatric glycerin suppositories can be given but you'll want to double check the label to ensure that they only contain glycerin The following link is to the sort of suppository I mean.
I typically only advise their use when oral products are ineffective.
I wouldn't give him any specific drugs for pain per se since cats are sensitive to most of them which are available over the counter; they might make the situation worse which is the last thing I'd want to do.
I hope this helps and that you're successful in relieving his constipation problem. Deb