Cat constipated and not pooping

I took my 11 year old male cat to the ER Sunday night because he cried out while pooping.

They did x rays and said that he was constipated (I have copies of the x rays but not sure how to post them) but also seemed more concerned about potential urinary issues despite his urinalysis coming back fine and him not having any issues peeing. They gave sub-q fluids and a shot of buprenorphine to help with “bladder pain.” (to be fair, they did also do an ultrasound and said he most likely has FIC based on what they saw. They were expecting the urinalysis to show crystals, but it didn’t)

It’s now Tuesday morning and he has not pooped since before I took him to the hospital on Sunday night at approx. 9pm.

I started giving him 1/8th tsp of Miralax yesterday morning. Is there anything else I can do? He won’t eat pumpkin. How long should I wait before taking him to the vet?