What do you mean, by "sad", please describe what gives you that impression.
Sitting hunched up and not doing much? That could be a symptom of pain, not of being sad.
Does he eat and poop properly? Comes running for a treat?
If not, I would see the vet asap, meanwhile giving Simethicone Drops (Baby gas drops), food he likes most (no sugary treats though, safe things like apple leaves, narrowleave plantain..., not too much if he isn't used to any green stuff, just to get him eating) and a gentle belly massage, the most common problem with rabbits are digestion issues, and should be adressed as soon as possible. They hide problems real well, when we notice it's already quite bad, and if it gets worse it can spiral out of hand quickly.