The former Scotland stand-off, who retired midway through the season to take up a role within Glasgow Warriors' pro-academy programme, will work alongside head coach Fergus Pringle and assistant coach Sean Kennedy during the tournament. Fellow former Warrior and Scotland international Henry Pyrgos has also joined the coaching group.
Weir said the support of Glasgow Warriors head coach Franco Smith had been crucial in helping him make the move into coaching.
"The way that Franco's dealt with me in the last couple of years has been brilliant," said Weir. "Having those open discussions and being really transparent and honest was great.
"I felt like when this job came up, he was so excited for me to apply for it, even though it might have put his roster under a little bit of stress for a little bit of time before they could recruit someone else in the summer. I believe so highly in his playing methods and the structures that he's put here for the last four years."
The Scotland U20 role marks another step in Weir's coaching development after beginning work with Glasgow's academy players earlier this year.
"I think coaching was always going to be my next step in my career. I've got a huge passion for it. I think naturally, as my career was going forward and getting to the later stages, that ability to coach people in a controlled setting sparked something inside me that I wanted to continue. When the academy job came up at Glasgow, even though it was mid-season for me, I didn't want to let it slip."
Pyrgos, who is currently coaching within Edinburgh Rugby's academy set-up, will also join the Scotland U20 staff as part of Scottish Rugby's performance pathway.
"I love the game, I'm really passionate about it," said Pyrgos. "I took a little bit of a detour to get to this position but I'm really looking forward to the future and it's a great role that I've got now.
"We've got some really high-level players in our academy. It's obviously super competitive and they're at a really key stage in their careers. They want to kick on and become professional rugby players, elite players. So it's nice from my side as I've been through that journey and now the other side, so I'm doing everything I can to support them on their journey.
"On the field, I feel I can add a lot of value to them. Those small conversations around high-performance behaviours and things like that will really help take their game to the next level."
Scottish Rugby Head of Performance Pathways Alastair Kellock believes both former internationals will make a significant contribution to the programme.
"As ex-players, Henry and Duncan know an awful lot about the game and what it takes to come through that pathway system. They are coaches, but also they've been tasked with mentoring the players, making sure that the players understand that it's more than just what they do on the pitch for training or games.
"Duncan was obviously still a player and chose to step away slightly early with the blessing of Glasgow Warriors and Franco helped him with that decision as well. He also, in the last couple of years as a player, has worked with these academy players. His personality means that all the players want to be alongside him, they want to be hearing from him and they'll take everything he says very seriously.
"Henry's journey has been slightly different. He stepped away and went into the finance world and kept his coaching going at the University of Edinburgh. Henry's a deep thinker of the game, he always was when he played as well. They're different characters, but both bring that slightly different dynamic to their coaching. Henry's depth and knowledge will be something that will set him in great stead as far as his long-term coaching career is concerned.
"Henry and Duncan's role at the Junior World Championship this year will be to support Fergus's team. It's brilliant from our point of view to be able to develop these coaches that are in our system. They work with the individual players and the collective group in the academy all year long, but to get some team coaching will massively important in their development."
Scotland U20 Fixtures
Saturday 27 June: Scotland v Italy, 10.00am
Thursday 2 July: Scotland v New Zealand, 3.00pm
Tuesday 7 July: Scotland v Japan, 12.30pm