How do I console my dog that has lost his bestfriend of 3 yrs?

I bought our Boxer just over 3 yrs ago it was in November a month later on Christmas night a kitten came running by us and became friends instantly with him. They have played well together despite the fact that our boxer grew to be 80 pounds of muscle while Kitty only grew to be 16 pounds. Braddex our Boxer was protective of Kitty, cleaning him and snuggling him. Well just the other day Kitty had to be put to sleep due to uncontrollable cirstances he was very sick. Braddex continues to look for his friend and has been whimpering alot. I just don't know what to do for him. If you have any questions just ask. Thanks! Please help.