If your 6yrs is able to move his foot and there is no significant swelling then it is less likely that he is suffering from fracture
.It must be either a sprain
or soft tissue injury
.You need to give him painkillers , warm fomentation and apply muscle relaxant
gel on the area of injury.
If the pain is excruciating then it would be advisable to get him examined and X-rayed to rule out fracture or hair-line fractures.