The 22-year-old was signed last summer for the Cobblers by then boss Gary Johnson who has snapped up the winger for a second time following his return to Huish Park late last season.
Young told BBC Radio Northampton at the time of his move to Sixfields: "Hopefully I can create goals for the forwards we're going to have at the club this season.
"Defensively we are looking really strong and we're creating goals in training and now we just need to put it into practice in games."
He was allowed to leave by Aidy Boothroyd at the end of the season.
Former loanee Sam Togwell meanwhile has signed for Chesterfield following his exit from Scunthorpe United.