I'd ring the ward, that sounds like something the surgeon/ consultant should know about and be able to comment on.
If they won't engage with it then get an emergency appointment with your GP first thing tomorrow. They may send you back to hospital but will hopefully deal the bureaucracy / hoop jumping for you.
I wouldn't be reassured by a blanket 'oh it's fine', but get more specific advice.
I say that as I almost died from septicaemia following a routine operation, because the ward fobbed off my parents when they phoned and told them that the symptoms were too unimportant for them to have bothered the ward with and to please don't call them back. After I almost died and had many days of emergency treatment, none of which needed to happen if it had been picked up on earlier, the ward again fobbed off a query about their behaviour by saying it had been after 48hrs after discharge and it wasn't their job to help. Except they didn't tell my very worried parents that on the phone ... Apparently we were all expected to magically know the ward saying 'it's normal and you are wasting our time, we have properly ill people to deal with here' actually meant 'we can't help you now as its too long after discharge but that doesn't sound right so please call xxx who can help with an out of hours emergency'. Grrrr. Oh and I think when pressed again they said there was a shift change etc etc.
Now, Im sure your little boy doesn't have anything similar and I don't mean to imply he has, but he might have an infection that needs prompt treatment, or the op needs a quick adjustment of some kind from the consultant.
And it's always better to be sure, especially if there is any confusion about who's remit it is to help.
Good luck 