Thank you for your query at DoctorSpring.com
I understand your concern.
Abdominal pain and diarrhea, provided it preceded the fever or occured together, mostly indicates acute gastroenteritis. The neck pain could be a part of the generalised myalgia. So it does not seem to be meningitis. Moreover one characteristic feature in meningitis is projectile vomiting, and if that is not present then it is highly unlikely to be meningitis.
It's a good thing that his fever has already resolved, so mostly such acute symptoms are due to a viral infection.
However you need to watch out for the development of projectile vomiting. In case that happens you can get a physician opinion to get all the necessary tests done immediately.
For his neck pain as of now, i advise Tab. Advil twice a day, it should resolve in a couple of days time.