Callum is a Scotland U20 international and recently made his Glasgow Warriors debut in October against Zebre in the Guinness PRO12.
After arriving in London he will be reacquainted with fellow age-grade internationalists Cameron Fenton, Ally Miller, Hugh Fraser, Tom Galbraith and Patrick Kelly.
Also at the club are Scottish Rugby's Head of International Age-Grade Rugby, Sean Lineen, who has taken up the role of Director of Rugby, and former Scotland U20s Coach John Dalziel as Forwards Coach.
The 19-year-old is looking forward to a new challenge with the Scots. "I'm really excited to get stuck in and continue my development with Scottish.
"I have played with some of the guys before at Scotland age group level and I'm looking forward to getting straight in to action in a big derby match against Richmond on Friday."