Thanks for your Query.foods containing omega-3/6/9 clubbed with some brain exercise will help the child to improve his/her memory.
foods that are Rich in omega 3/6/9 are cod liver oil,flax seeds,avacado.
also add some antioxidants like SOD,raisins,curry leaves.
SOME HERBS Like BRAHMI
,VAYAMBHU will also meet your purpose.
Make him to do YOGA regularly.there are specific yoga postures like vrikshasana,veerabhadrasana/pranayams and meditation
for the same.This will contribute to integrity of mind.also exercise brain through games puzzlebox.CHESS,etc.
remember ''anubhuta vishaya sampramosha smrithi"
memory is all about retaining the Input.
good luck !!
M.D & Consultant-Naturopathy