The former Saints academy graduate returns to the club to join the U21s as a player-coach.
Reeves said: "It's a privilege to be back and an honour to be thought of for the role.
"It's a great opportunity for me to be back in the building and it doesn't come around often, so I want to thank Olly (Lancashire) and Russell (Martin) for their help in the process.
"It's a great opportunity to help these boys and learn off people like Olly, Russell, and Simon (Rusk), and I've already learnt a lot already - I'm looking forward to being around the Under-21s and hopefully help make them better people and better players."
Olly Lancashire, now the Academy's Head of Player Strategy, also told the club's website: "Having been in this role myself, I was asked to make a shortlist of potential people to take it on and Ben was top of that list so it's great to be able to welcome him back.
"It's worked out really well that Ben's played at MK Dons with Russell, Simon Rusk has had the same position himself at Brighton, so everything has come together at the right time.
"I'm really confident that Ben will have a positive impact on the staff, and more importantly the players, and it's important we have another Academy graduate returning to the club - I'm sure he'll have a big impact and we're looking forward to working together this season."