My child has recently started to wake up terrified, with glassy eyes and crying. Is he having nightmares? After some time, he just goes back to sleep.

has there been any changes in his routine? talk to him if he is sacred of something or someone, night terrors do occur but check external factors which might be causing it also ensure your kid isnt watching scary stuff on mobile/tv and refrain your kid from watching anything on tv/mobile atlest 2 hrs before naptime.