Hi
You might try backing him off the water and go to just ice chips. One at a time if that's what he has to do. Keep him well supplied.
Is he passing gas yet? Can he hear or feel his stomach rumbling yet? If you have access to a stethescope, you might listen to his lower abdomen for the presence of bowels sounds.
When we are given general anesthesia, coupled with bowel surgery like your son had, it takes awhile for things to start moving down there again. The anesthesia paralyses it for a time.
So, try the ice chips, really slow and steady....like one ever 5-10 minutes. See how that works.
If it still doesn't help, it's time to call his doctor again. He may prescribe an anti-nausea pill or suppository he can use.
Hope that helps,
Kerry