This question already has an answer here:
In short: Do totem barbarian features continue after they are polymorphed?
In long: The 6th level bear totem barbarian feature doubles carrying capacity. Carrying capacity is your strength times 15 in pounds. With strength of 16, he can carry 240 pounds. Then double it for the bear totem to 480 pounds.
If this barbarian was polymorphed into a polar bear, they would have strength 20, so a carrying capacity of 300 pounds. If bear totem still applies, this doubles to 600 pounds.
Unknown: I don't know how or if, size affects this as the barbarian is medium and the bear is large.