Jennifer,
There are two things you mention that are extremely important to take care of and they are his inability to keep much down and also his decreased water intake. His not drinking much could be partly due to his not keeping much down, and so someone should talk with his doctor or his doctor's nurse to let them know of his situation. There could indeed be other things going on, but much of it may be what I just mentioned. These are not small things, and the sooner you or someone speaks to his docotr the better.
With him having had his surgery such a short time ago it is not out of line to call in the evening, most doctors expect to occasionally get calls from their surgical patients at night. They would prefer you call them now, rather than wait and perhaps have the patient become much sicker in the meantime.
You don't mention if he was doing better in the hospital and when he first arrived home, if this is different then it was then that makes it even more important to get answers now rather than wait. I'll keep you in my prayers and let us know how he's doing.
God Bless,
Lou Graham