There is no United Nations Bureau of Moose Weights, but the best source I’ve found is Valerius Geist’s Deer of the World. The heaviest weight Geist could find for an Alaskan bull was 1,572 pounds.
Outdoor Skills – Hunting Q&A
I've hunted moose in various places, and wherever I've gone, the animals' live weight is an often discussed topic. What's officially considered the live weight of Alaska-Yukon moose?