It is not too late. Make sure he keeps warm (f ex by giving him a warm waterbottle in a towel). It is extremely important that he is kept warm.
You should support feed him if he isn’t really eating. That he has pooped is great, but he needs food in his tummy continously. Oxbow’s Critical Care is a practical and easy-to-use support feed for poorly buns, there are other brands as well. It is a powder that you mix with water and feed by syringe. You should feed him every 3-4 hours for starters.
If you cant get Critical Care, you can soak pellets in water and feed the mush in a syringe.
It is good if you can get him to move around a bit. If he’s had an opioid painkiller he will be very drowsy from that, which is normal.
What meds were you given?
Here is a very good article about GI stasis in rabbits. http://www.bio.miami.edu/hare/ileus.html
Here is an instruction video on how to help a bun with gas pain (GI stasis as a rule causes gas pain):
https://youtu.be/LbyC6CWbm5M
Please keep us posted!