Relax. Relaxing will help your child to focus. Try having the child lie down and breathe slowly and deeply for five minutes. Small children can relax in your lap.
Have your child close his eyes and, after a few seconds, ask him to tell you what he sees as he sees it. As the images and associated emotions come up and he verbalizes them, he is, figuratively speaking, clearing his mental slate. This helps him prepare to record images again with his innate photographic memory.
Practice by playing games. Once your child is relaxed and has "downloaded" stored information to make room for more, lay out four flash cards with pictures of familiar objects on them. Let him glance at them for just a second, then take them away and ask him to recall and name as many images as he can.
Continue playing, increasing the number of cards as his recall ability improves. There's no need to insist that he can remember every card before increasing the number. Every time he gets anywhere close to recalling all of them, lay out at least half as many more cards.
Vary your practice by having your child draw pictures of the images he recalls instead of naming them. You can also introduce him to the adult memory-training techniques of associating pictures. For example, teach him that instead of recalling an apple, a book, a baby and a cat separately, he can form an image of something surprising that connects all four objects like a baby reading a book about a cat that juggled apples. This makes it easier to remember a large number and variety of disparate images.