Complex sentence: His first major international tournament was at the 1948 Olympic Games as a lightweight boxer, which was followed up as a Canadian team member of the 1950 British Empire Games (now called the Commonwealth Games).
Split into simpler sentences: His first major international tournament was at the 1948 Olympic Games as a lightweight boxer. That was followed-up as a Canadian team member of the 1950 British Empire Games (now called the Commonwealth Games).