I'm sorry to hear that memory problems may be affecting your son's school performance. Here are some resources I hope will provide insights for you to help resolve the issue:
 
Attention is Essential for Most Learning and Memory
http://www.education.com/reference/article/attention-essential-learning-memory/
 
Memory and the Brain (page 2 covers poor memory issues; page 3 & 4 provides info about using senses to promote learning)
http://www.education.com/reference/article/Ref_Memory_Brain/
 
Are You a Creator? An Essay for Kids! (includes checklist to help determine if child is a visual spatial learner, and if so, offers tips to excel in school)
http://www.education.com/reference/article/Ref_You_Creator_Essay/
 
Using Visual-Spatial Strengths to Memorize New Material
http://www.education.com/reference/article/Ref_Using_Visual_Spatial/
 
Learning How to Study
http://www.education.com/reference/article/learning-how-to-study/
 
The Role of Parents in Children's School Achievement
http://www.education.com/reference/article/role-parent-children-school-achievement/
 
Organizing Your Preteen: Tools for Success
http://www.education.com/magazine/article/Organizing_Your_Preteen_Tools/
 
And here is a fun math-related activity you might like to try with your son:
The Mean, Median, and Mode Card Game
http://www.education.com/activity/article/Mean_Mode_fifth/
 
Good luck - I hope you are successful in improving his school performance.