Canceling a creature spell during combat
If he has said that he takes the life loss from Shaman of the Pack then the spell has already resolved, entered the battlefield, the enter-the-battlefield-trigger has been put on the stack and resolved. It is then way too late to decide to counter the Shaman.

If he had not yet acknowledged the life loss from the Shaman verbally or by obvious gesture, then he still retains priority and you cannot decide to move to combat.

The important thing is that he acknowledged the loss of life.