Answer: The "him" by King Claudius could have been referring to young Fortinbras or King Hamlet, his brother.
Explanation: If the king was referring to young Fortinbras as most of the court would have presumed, it would be an indication of his confidence in his kingdom's military prowess and would suggest pride and arrogant confidence.
On the other hand, if Claudius was referring to his brother King Hamlet,
it would be heartless of him which would suggest a wickedness in him. One reason to support this is young Hamlet's suspicion of foul play in his father's death as the scene draws to an end