Dion joined Chelsea's academy as an Under-13, having previously spent time with Fulham. He made his Under-18 debut during the 2011-12 season and started games at home to Crystal Palace and Southampton in the first half of the season. After Christmas, he began to make more of an impact in the team and scored his first goal at Under-18 level against Bristol City in April 2012.
In his first season as a scholar he became a regular in the heart of the Under-18 team and pushed on into NextGen Series and Under-21 contention by mid-season. He earned more Under-21 appearances the later the season went on with other contenders featuring more regularly in cup competitions, but he would likely have been involved on merit as well, enjoying a fine first year as a scholar.
2013-14 would prove an up and down campaign as he established himself as one of the key players at Under-18 level and began to make inroads into the Under-19 team, but when opportunities arose to step up to Dermot Drummy's Under-21 team he went down with injury and missed the last six weeks of the season which included the Finals of the Under-21 Premier League and the FA Youth Cup. He would ultimately be sidelined until late December.
Eventually fit to resume playing in March, Dion finished the season strongly and injury-free, promising a much better 2015-16 season. By and large that proved the case as he made 21 appearances from a possible 28, including 16 starts whilst shaking off the one or two slight knocks he picked up throughout the season. He also signed a new contract to keep him at Chelsea until the end of the 2017-18 season.