You should never feed a rabbit lettuce! It's a toxin to them, it not only makes them unhealthy, but I've read it completely alters their personality too. First thing I would do would be to cut out the lettuce. It also has no nutritional value to a rabbit, so it can't hurt to cut it out.

I'd also book him in for a vets apoointment, being a house bunny, there's all kind of things he could have eaten without you noticing, that could be making him feel out of sorts. It's strange for a rabbit to change so quickly without good reason.

Good luck.

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