McCallum started playing rugby with West of Scotland and the former Loretto School pupil has represented Scotland at U16, U18, U19 and U120 levels. The centre joins from Spanish side Complutence Cisneros.
McCallum has also represented Scotland at cricket at both U15 and U17 levels.
The centre was an intergral player during the 2019 U20 Six Nations where he started all but one fixture - capping off a stellar tournament with an impressive try in the away defeat to France.
McCallum's form continued during the World Rugby U20 Championship in Argentina, with the centre grabbing a memorable solo try in the pool stage deafeat to New Zealand.