The first time we left Willow at home and went on vacation for a few days (my parents looked after her), we came back and she shunned us for a week and a half. The worst thing that ever happened though was when we didn’t even leave the house – we were cleaning all day long and Willow saw us not spending time with her. That evening, we tried to stroke her and give her treats but she refused every treat, went straight to her cage and flopped repeatedly saying “I’m happy without you.” She barely hopped around too. I thought something was wrong because she was refusing her treats and not moving around but then we left the treat in her cage, left to go to bed, and when we came back into the room, she was happily hopping about her cage and her treat was gone. She saw us and went back to acting miserable.
Basically, if he’s still eating his food, drinking, and going to the bathroom normally, he’s fine. He’s just giving you a hard time because you left and he wants you to feel bad about it. Our bunny drinks a ton of water some weeks and not nearly as much other weeks. As long as you still see him drinking once in a while, I’m sure he’s fine.
As RightD said, it does get better overtime. My husband and I were in New Zealand for three weeks recently (we’ve had Willow for a year and a half now) and when we came back, she didn’t shun us much at all. She shunned my husband a bit (because he’s the one who works), but after a day or two, she was normal with him too. 