What Causes High Fever And Shivering In A Child?

My 2 year old son suddenly woke up with shakes, 104° fever (felt higher, but the thermometer only goes that high and read "high" because it tops out at 104°), and 150 bpm. Is this something I should be concerned about or just continue to treat with Tylenol and watch it? With Tylenol, his fever is down to 103°. Thank you.