Josefson played in his first IIHF sanctioned tournament when he was part of Sweden's team at the 2008 IIHF World U18 Championships. He managed to score a hat-trick in the first game against Belarus. He left the team in the morning before the fourth game due to an accident in which his father and uncle had been exposed to carbon monoxide poisoning. He made his second appearance at the U18 Championship the following year, at the 2009 IIHF World U18 Championships, where he also took the role as alternate captain. He managed to score three goals and seven points in six games. Josefson was a part of Sweden's national junior team at the 2009 World Junior Championships. He played in all of Sweden's six games but did not score any points. A few months later he yet again played for Sweden in the U18 Championship but could not help his team go further than fifth place in the tournament.