When he had spent (dapanēsantos autou). Genitive absolute. The verb is here used in a bad sense as in Jam 4:3. See note on dapanē Luk 14:28.
He (autos). Emphasis.
To be in want (hustereisthai). The verb is from husteros, 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.