Then he is extremely sinful.
Ibnul-Qayyim (rahimahullaah, d. 752H) stated,
“Whoever pays no attention to teaching his child that which will benefit him and he leaves him to waste away without care, then he is extremely sinful.
Most children are corrupted (and led to failure) due to their parents.
They pay no attention to them and leave off teaching them the obligations of the religion and the Sunan. So they ruin them whilst they are still children.
And they (the children) are not able to benefit themselves and their parents do not benefit them.”
Tuhfatul-Mawdood fee Ahkaamil-Mawlood, p.80