What Causes A Pins And Needles Sensation In A 4 Year Old Child?

My 4 year old son keeps getting pins and needles in his feet. It s been going on for a couple of mouths now. And seems to be getting worse. It uselly gets this in the middle of the night and during the day. For example today he was sitting nicely with me on a bus journey and it happened. I tock his shoe off and it took ages for it to go away. It was hurting him so much it made him cry.