Having a poor memory is not all that uncommon. One of my children is that way, and so was my father. They learn ways to maneuver around it. It does annoy my daughter though.
There are study techniques that really improve memory. I don't quite understand what you mean by short term memory. Even the people that appear to glance at a word, and know it the next day use techniques to do so. And some superbright people can literally do just that.
I think the main question is why post to an ADHD forum. If he has trouble memorizing because he can't concentrate (ADD/ADHD), that is very different than just having trouble memorizing. Although the techniques will work for both. Its just a lot more difficult and time consuming for the ADD/ADHD child. There is no magical diet that will help. A good nutritional diet is always important.
So essentially, I need more information so that I can help you. Do you suspect ADHD? What subject is he having trouble in? Has he always had this problem? Does he think the teacher is a jerk? Depending on what is going on I have some books, that I can recommend.