[@DaredevilSpider] Certainly, please bear in mind, this is an IC observation of your character, not my opinion of you.

Isa is a hunter. Jacob is smaller, probably means prey. Jacob is paranoid, almost certainly means prey.Jacob didn't eat anything, when everyone else did, probably means weak prey.(based on the fact that prey rarely eats the same things their predator does) Jacob ran off/away to his room at the first opportunity, probably means easy prey. Jacob rejoined the others, but waited for the group to acknowledge him instead of claiming his place, might mean prey. Isa is resigned to working with Jacob, only because he has no choice in the matter. However, he fully expects the group will have to carry Jacob's share of the load on top of their own loads; because Jacob has to date proven to be nothing more than a moody and cowardly brat in Isa's eyes. 

callously measured: Isa has measured Jacob's worth as a person based on little more than Jacob's appearance and initial impression, using his own experience as a hunter and trapper as the basis for his judgement of Jacob and Jacob's  personal character.
found wanting: Isa experience tells him that some qualities are better than others, Jacob lacks these qualities. (physical strength, personal pride, cunning, determination, and persistence)
then dismissed: Isa expects Jacob to die or be permanently incapacitated during the first dangerous/violent mission the team is sent on, and he's not going to be shedding any tears for Jacob when it happens. 

Does that make sense/help any?