Which son of Isaiah went with him to meet King Ahaz in Isaiah 7?

Answer: Shear-jashub

The LORD told Isaiah to go out with his son Shear-jashub to meet Ahaz at the end of the aqueduct of the upper pool.

“Then the LORD said to Isaiah, “Go out with your son Shear-jashub to meet Ahaz at the end of the aqueduct that feeds the upper pool, on the road to the Launderer’s Field,”— Isaiah 7:3 (BSB)