The prodigal in Elizabeth Bishop's poem is only metaphorically related to the Prodigal Son in the parable told by Jesus in the New Testament. This modern prodigal is evidently an alcoholic, which would explain how he had sunk to such a low condition that he was sleeping in a barn that reeked of manure.
But sometimes mornings after drinking bouts
(he hid the pints behind the two-by-fours)...
The first star of evening gave him warning that the farmer he worked for would be coming at dark to shut the cows and horses in the barn. So the first star of evening would warn him to appear sober and industrious and to hide whatever bottles he had not already hidden.