You have not mentioned the frequency of urine in a day and weight of your son.
If urine output is more than 2 litres/ sq m of body surface area/ 24 hrs, then it is to be called polyuria
First see if he drinks lot of fluids- that is one the reasons for increased urine output.
is not the only cause for pathologic poluria. There are some more entities to be ruled output.
You might have to visit a Pediatrician- who will advise you to measure 24 hr urine output and then decide further.
If along with polyuria, if he is not gaining weight, if he is irritable, has bed wetting
at night time etc, it might indicate underlying problem and the need to investigate him.
Memory deficit may not be related to this. Both genetic and environmental influences have a role in one`s memory. Repetition is the key for enhancing memory.
Read, reread, register, remember, recollect- all these 5 should be followed as a routine- to have a good memory.
Give him healthy diet- which includes lot of fresh fruits, vegetables, dry fruits and nuts than junks.
However reading and memorising lessons is not the only thing of student`s life. Just explore his talents, field of interest and encourage him to follow his passion.
Hope my answer helps you.
Wish your son good health.