He went astray, son of a fool. ذَهَبَ فِي ضُلِّ بْنِ أُلٍّ‏

If he persisted in falsehood, it is said: He went astray, astray, and astray, if he went astray without right.
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This proverb is used to describe someone who has gone too far in their error and stubbornness, embarking on a path of misguidance without return. Some of the closest English equivalents are:
“He has gone off the deep end.”
Meaning: He has gone too far into error, madness, or extremism.
“To go down the wrong road.”
Meaning: To take the wrong path.
“He is deep in error.”
Meaning: He is deeply immersed in error and misguidance.
“To persist in folly.”
Meaning: He persists in foolishness and falsehood.
“He has lost his way completely.”
Meaning: He has gone completely astray.
The closest expression to the spirit of the Arabic proverb is:
“He has gone far astray.”
“He has gone too far into misguidance.”
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إذا ركبَ رأسَه في الباطل، يقال‏:‏ ذهب في الضَّلاَل والألال، والضلال والتلال، إذا ذهب في غير حق‏.‏