I also love Norwich, because it's small and quiet and historic.
Bug parc really is quite far from Norwich though, a taxi would cost a fortune - if DS can tolerate the bus then bus to Dereham and then taxi might be better.
Norwich museum has a fairly large Natural History section with lots of bugs and they do a quiet / sensory / autism morning on Sundays.
Roar is not loud it is just a big park with big dinosaur models. It has a high ropes course for an additional fee, a 4x4 trailer tour to see the deer, a water splash pd which probably won't be open this time of year, a soft play area which DS is probably too big for but that's the only loud section and easy to avoid.
Bewilderwood is fabulous, it's basically a big adventure playground in the woods but gives with the Bewilderwood books.
I'd check opening times for all as it's not unusual for places to be shut midweek.
That train ticket looks expensive if you haven't already got a Railcard the friends and family one might be perfect.