Lord, have mercy on my son,.... He addressed him with great marks of honour and respect, not only by gesture, but by words; he craves mercy, pity, and compassion; for the case he had to present, was a miserable one; and his earnestness and importunity he hoped might be excused, since it was for a child of his own.
Luke adds "for he is mine only child"; and therefore his affection for him must be thought to be very strong, and he greatly concerned for its grievous affliction, and earnestly desirous of its health and life.
For he is lunatic: not a mad man, but troubled with the epileptic disease; upon which, as on madness or lunacy, the changes and full of the moon have an influence: hence the next clause,and sore vexed, is rendered in the Arabic version, "and sore vexed at the beginning of full moons"; at which times, he had very grievous and frequent fits of his disorder:for ofttimes he falleth into the fire, and oft into the water: which shows it to be the "epilepsy", or "falling sickness", he was afflicted with; which, whenever it seized him, whether by the fireside, or by the side of a river or brook, or any place of water, or in any other dangerous situation, he fell into it, not being able to help himself, or avoid any danger to which he was exposed.
A larger account of this child's disorder, and of the circumstances of his cure, are related by Mr 9:17 where this case will be more fully considered. See Gill on "Mr 9:17".