There wouldn’t be any major change in the food your child consumes at his age. However, you can do small-scale moderation in his food, like feeding him frequently less liquid-based food, for eg, juice, soda, & milk.
Your kid is also likely to experience constipation which is quite a common issue post-surgery. Use of anaesthesia and pain medicine leads to the cause of constipation. Hence it is suggested to provide plenty of liquid to your child after surgery. If there’s any problem with the bowel movement which takes more than 48 hours, then seek immediate help from doctors.