My child is scratching his butt, is that a sure sign of pinworm?

No. But it could be.Look for the worms, coming out at nite.They look like little moving white threads.A stool sample to the lab would accurately identify them.Scratching there also comes from irritaion caused by reactions to t.P., poor hygiene etc.
No. Many children with pinworm infections have no symptoms and may never be aware of the infection, but parents notice the worms in the diaper or toilet after a bowel movement. Itching around the anus associated with scratching is a symptom, sometimes associated with restless sleep (itching is often worse at night). Local irritation and dry skin can also cause discomfort that responds to scratching.