As has already been said, sounds like he's been diagnosed with E.C. What has the vet told you to do? He's on medication? What has he been given? He needs to take the full dose prescribed to him. Do you know how to give it directly into a rabbit's mouth? Just behind the front teeth there's a gap. You can squeeze the syringe into that gap. Do it slowly to give him a chance to take it, you don't want to squeeze it all at once and choke him. Keep an eye on how much he's eating, if he stops completely or is having very very little he could be going into GI stasis. If he is then he needs to see an emergency vet now. If he's getting worse then again, I would say he needs to see an emergency vet now. Tomorrow could be too late. Trust me, I've learnt this the hard way.
Please try not to panic. I know it's hard to see him like that, but you need to stay calm to help him. He needs you.
The brand name usually used over here is Panacur. Each syringe contains enough for nine doses. It's reasonably easy to get, you don't need a prescription from most places. This was the cheapest one I could find when I ordered some a few weeks back:
http://www.vetuk.co.uk/rabbit-supplies-rabbit-wormers-c-649_651/panacur-rabbit-oral-paste-5g-p-1105 but you'll need it urgently so I would ask your vet for it if you haven't already been given it. You can order it online cheaper next time (don't get it on ebay though, there are some fake medications popping up on there)
As others have said, EC isn't a death sentance for all rabbits, it's very possible he'll be okay but he needs the right treatment.
I would offer him a shallow bowl of water instead of a bottle, the bottle will be too difficult for him to use right now. You may need to help him with it. What MimzMum said is the best idea, wash fresh vegetables for him, put them in a dish but don't shake the water off. The more water on the vegetables the more water he's taking in. You need to make sure he's not getting dehydrated. You mentioned lettuce, what type is it? Bunnies can't have iceberg lettuce, it's dangerous for them. If you have any fresh herbs such as parsley, coriander etc try offering those too. Sometimes more fragrant foods like herbs can tempt a bunny to eat. Again you can wash these to increase water intake.
I'm sure he'll be okay, please try not to panic. I know how hard that is. But you need to stay calm for him.
If he goes downhill at all, he needs to go straight to the vet. If you need to find an emergency vet in your area try here:
https://www.vets-now.com/ they can also offer advice free on the phone. I've used them a couple of times and they are absoloutely brilliant, they saved Lunar's life and Cody's but are obviously a lot more expensive than a normal vet. Just an option if you find you need it and your regular vet is closed.