What Causes Low Sodium Levels In Children With Epilepsy?

My son has partial complex seizures , controlled by tegretol 500mg /day + frisium b.d. # years back had a car accident where faciomaxillary bones and mandible had fractures. several operations have improved his condition. Last week had to go to orthodontist for 3 teeth implants. primary operation is done and is due for implant after 2 months. After the recent operation he was on augmentin1Gm b.d. + bufen and codein for pain relief for 5 days. now he has developed urinary incontinence while bending during prayer and nocturnal too . V,embrassing for him as he is 21 years old now. since his epilepsy diagnosed , i found his Na always around 135 ( low normal ). what could be the probale reson for his incontonence at this age and what furthuer to do ?