Lisa, he's feeling pretty low, but with a disease which affects the liver and gall ducts, that's to be expected. If he wasn't eating or drinking, I'd be more inclined to agree it may be time to let him go, but I think it's worth letting the vet take a look, and see what they think. It was just yesterday when he was more relaxed and rolled over to show you his moggnifikatt tummy. With an illness like this, it would seem likely he'll have good days and bad days, and it isn't going to heal overnight.
It's easy for me to say, but believe me, I'm not trying to judge. Any decision you take will have Leo's welfare at its very heart. Just I can see that this is tearing you apart, and when you're right in the thick of a situation like this, it's easy to lose yourself in the anxiety of seeing your lovely little lad in pain. A change of meds may be worth the attempt.