Why does my child get anxiety in the evenings?

Depends on his age,: personal and family history of medical and psychiatric illnesses, exposure to trauma & home, school & community environments. If it's normal separation anxiety at 9-11 mos., or night-time fears at 3-4 yrs., attending to his crying or allowing him to sleep in your bed reinforces his behavior. At a consistent bedtime, read, cuddle, brush teeth & put him in his bed to fall asleep. See comment.