When he had spent (δαπανησαντος αυτου). Genitive absolute. The verb is here used in a bad sense as in James 4:3. See on δαπανη Luke 14:28.

He (αυτος). Emphasis.

To be in want (υστερεισθα). The verb is from υστερος, behind or later (comparative). We use "fall behind" (Vincent) of one in straitened circumstances. Plummer notes the coincidences of Providence. The very land was in a famine when the boy had spent all.