With so many school and after school activities, your child at this point is constantly exposed to learning new concepts. This also creates the need to remember more things and hence in order to meet the new challenges, your child has to improve his memory abilities naturally.
What you need to know
During the process of memorizing things, you may observe your child will remember things that he finds interesting. And when he finds himself forgetting things that he is expected to remember, he is forced into realizing that memory requires work. For instance, he may be aware of an upcoming basketball match but may forget about his Olympiad exam dates. Writing stuff down in such cases helps. He can keep a calendar of events like Olympiad exams, Sports Day, a visit to the museum etc, that are important to him. This way your child understands that by noting things down he will remember them better and not miss out on important events.
What you can do
Another way you can work with your child to build his memory is to break the task into chunks. For instance, if he has to memorize a poem, break it down in parts. If he has a week to memorize the poem, work on learning the poem part by part daily rather than doing the entire poem in one go.
Visualization helps. For example, you have asked him to plan for his birthday party. Get your child to visualize what all he will need. Keeping in mind the various birthday parties he has attended, he can list out things needed for his own party, thereby working on his memory.
As a family rule, create a place for everything, be it your car keys, wallet, wrist watch or your child's toys, pencils, ID card etc. Seeing you keeping things at their respective place, your child will model the same behaviour. Using the above strategies will help him to expand his ability to remember, which is an important part of his cognitive growth. As he grows, his memory capacity will continue to increase with knowledge and experience.