No. Go to a child health nurse. He might just have an intolerance, or need more hind milk.

Do you have a post operation appointment ?
When your doctors can’t give you an answer we should be hesitant to give you an answer too.
A poo chart can give you an indication in which direction you can think;
https://www.25doctors.com/poop-chart

If your doctor isn’t concerned, it may be nothing to worry about. Baby poop changes colour and consistency depending on what they’re eating, drinking and how they’re feeling.

I would try to contact the Surgeon who performed the surgery and ask if that is normal.
Maybe now that he has had the surgery he may need a formula change. If a Dr. can’t help you perhaps talk to a pharmacist. Sometimes they have more answers than a Dr. does. I have found this to be the case on a few occasions.