There are numerous ways to improve memory skills with any student at any age.  The memory can be worked just like a muscle.  You start slowly and build on whatever is already there.

One thing you can do is figure out how many images your child is able to hold in his mind.  Hold up a drawing of three random shapes, such as a heart, a star and a flower.  Have your son look at it for about ten seconds.  Take it away and have him draw the shapes.  If he is unable to draw the shapes, have him tell you the shapes.  If he can't do three shapes, bump it back down to two shapes.  If he did three with ease, build up to four shapes.  I like seven year olds to be able to hold four to five images in their minds. 

Working memory can come along once visual and auditory memory are in place.  You can do the same kind of activity with your child for auditory memory - just use random words.  Working memory involves manipulating these images or auditory stimulus, so I don't have any quick easy fixes for that.  We use many different approaches at our centers that increase the student's working memory, but they are a bit more difficult to explain in a short email.

I hope this helps, and I wish you the best of luck with your son.