My son whose age is 15 years, usually gets night fall while sleeping. He usually gets night fall (ehtelam) twice a week. In my opinion it is frequent night fall (ehtelam) and his health could be impaired badly due to it. Although my son seems to be healthy.
The night fall (ehtelam) is the sign that your son has entered in the stage of puberty. These night fall (ehtelam) are natural and never be harmful for any person. Whenever; a body gets enough maturity mentally and physically, then his frequency of night fall (ehtelam) declines. After hat he stays at a stage.
Your son's problem is not out of normality. You should not be worried. Also you should explain your son in this regard that this night fall (ehtelam) is almost a natural phenomenon. Some people get more while some get less.