Your 6 yr.old son has this problem that means his diet & life style is mismanaged.
Black heads on facial skin denotes ill health of skin & improper digestion.
See carefully what the food he ingests is digested properly.diet should include fibers ,proteins
,carbohydrates, vitamines ,minerals in a balanced manner.leafy vegetables,fruits,cereals ,pulses,milk should be included.For enough liquid content 1 glass of water every hourly or fruit juice can be added.
Removal of blackheads is necessary.Facepack of honey,rose water 1tsf,lime juice 1/2 tsf,turmeric & chana dal powder daily is a good remedy.