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His problem may be due to stress
of surgery or due to some post operative pain. The important thing is too maintain his nutrition. If he is not eating properly then some IV fluids can be given. Stress and depressive symptoms usually resolve after few weeks of surgery.
For suspected pain problem you meet some veterinary surgeon.
I hope this information has been both informative and helpful for you.
Wish you Good Health.
Dr. Ashish Mittal