Is the weather getting warmer where you are? Rabbits don’t take heat well and they will be less active when it is hotter. You can find cooling tips here in the BUNNY INFO section.
If a rabbit is pressing his stomach into the floor, that usually means that he has gas. You may also see him shifting around, trying to get comfortable if he has gas. You didn’t mention any of that, so it might not be gas. If it was, you can use infant simethicone drops.
When your bunny does hop around, does he do so normally? He isn’t favoring any of his legs, is he? That would indicate a possible injury.
The fact that his eating, drinking, and pooping is normal is a good sign. It may be the temperature that is affecting him. I don’t know if the new dog in the household would be affecting him or not. You would see if he was scared or stressed by the dog, and you’re obviously not seeing that.
If anything changes and he is not eating or pooping as much, you should take him to a vet.