Howie- Your post brought tears to my eyes- what a great dad you are to Lenny! Much of what you describe is familiar to me - my 17 year old girl also has liver disease (for a few years now) so I empathize with what you're feeling.
As your vet may have explained, there are many types of liver disease. Do they know which specific one Lenny has? Some conditions can be cured, most are treatable. One very important thing to keep in mind - the liver is the only organ in the body which can regenerate itself, provided there are still some healthy cells left. It's an amazing organ. Remembering this fact has gotten me through more tough times than I care to recall.
My preference would be to find a fairly large animal hospital with board certified internists, which provides 24 hour care. I'd be very reluctant to leave Lenny alone all night.
Vomiting is a pretty typical side effect of liver disease, unfortunately. My girl is also on Pepcid which helped quite a bit. Lenny is on lots of meds right now, so it's possible that they're not agreeing with him, and could be the cause of the vomiting. Antibiotics are tough on the digestive system, and often cause vomiting, diarrhea, or both, which in turn leads to inappetance. Speak with your vet about trying a different antibiotic that might be easier on Lenny's system.
It's good that Lenny's being given fluids, that's important. He also needs to eat. Since Lenny wants wet food, (which I personally feel is preferable to dry but you have to do whatever works) I would give it to him. I'm not a vet, but my own feeling is that with all the meds Lenny's on, the chances are greater that it's one of them or the combination of them that's making him sick, not the food. If the food never made him sick before, why would it bother him now? Just make sure it's high quality food - no by-products, nothing artificial. Colors and preservatives are just more junk that his already compromised liver has to process out. If he's not eating enough on his own, assist feed him. He's already got liver disease, you don't want to run the risk of hepatic lipidosis which is a serious condition of the liver caused by lack of eating or not eating enough. I've been assist feeding my girl for several months - it made a world of difference. This site will help you: www.assistfeed.com
Keep in mind that small, frequent meals are easier for the liver to handle. Consider adding a digestive enzyme to his food, like Prozyme, to help his body utilize the nutrients from the food.
Ask the vet to give you injectable B vitamin complex - very good for liver patients and it will sometimes help spark appetite. My cat also gets fluids, and I inject 1cc into the venoset port once per week, per my vet.
You said that Lenny had a bout of pancreatitis - maybe that's a chronic problem which could also be causing him to feel lousy. It can be difficult to diagnose - hopefully it was just the one time.
There are very beneficial supplements that you can use with Lenny. One is Denosyl, which was suggested by my vet. It's a human supplement, Sam-e, which you may have heard of. Denosyl is made specifically for cats and dogs. My kitty gets one per day - it has to be taken on an empty stomach. I get it through entirelypets.com, about $30 cheaper than what my animal hospital gets for it. Ask you vet about Denosyl, and also another product made by the same company called Marin. Marin contains silymarin (from milk thistle) which is very good for the liver. What's especially good about Marin is that the silymarin is combined with phosphatidycholine, which helps the body absorb the silymarin. My only reservation with this product is that is uses a very low dose of silymarin. I use a much higher dose with my cat.
How long has Lenny been on pred? When was his last bloodwork? Does it seem that the pred has been effective in decreasing the liver enzymes? My cat was only on pred a short while - ironically it seemed to spike her enzymes very high- ALT was over 1100! All the vets swore the pred wouldn't have done that, but I discontinued it (weaning her off - you never stop pred abruptly) and the enzymes went down, though never back to normal levels.
Once you get Lenny stabilized, speak with your vet about I-131 for his thyroid. It's radio-iodine therapy, and it's amazing. One of my cats had this therapy a few months ago and I'm extremely happy with the results. It's not cheap (is there anything vets do that is? LOL) but it's safe, non-invasive and targets only the thyroid gland, leaving other organs unaffected. The downside is that Lenny will have to stay at the vet's at least several days (mine had to spend 4) - the length of time varies depending on where you live. The good news is that his thyroid will be back to normal and you won't have to worry about giving him meds for that.
I know this is an extremely long post - I hope some of it will help. Try to be cheerful around Lenny - he needs you to be strong for him and reassuring.
Hang in there - all the best to you, your wife, and your sweet boy Lenny.