4.5 hour drive tomorrow with a sick cat. Please help!

12/19: 3.5 hour drive-seemed fine
12/21: intro of new kitten, bfs (indy), little hissing, playing and cuddling by 12/22
12/22: vomit seen from cat grass. Grass pulled, vomit stopped.
12/27: 4.5 hour drive with kitten (indy) and cat (telson), both slept 85% of the time
12/28: bf buys catnip plant, even at my behest, vomiting (telson) begins again and stops when catnip is pulled.
1/1: new dry adult food introduced, mixed with dry kitten old food.
1/2: lethargic cat (telson) noticed, not eating or drinking. Taken to local vet. X-rays done, no noticeable blockage, but has gas build up in intestines. Megacolon and blockage cannot be ruled out. Told to watch at home. Eats a little wet food at home, goes back to hiding.
1/3: kitten (Indy) presents diarrhea, cat (telson) taken to 2nd vet 1.5 hours away (closest petsmart). More X-rays done as well as blood work and pain shot. Blood is "beautiful" and gas and poo have moved in intestines. 104.6 temp. Given feliway diffuser and Rx wet food and alcohol on paw pads. No meds given for gas due to lack of eating.
1/4: noon wake up with cat (telson) on bed, purring. Ate some Rx food and drank some water. Couldn't feel a temp. Then spent most of the day out of the closet and roaming.
Now, he is hiding and has no interest in even canned tuna. I don't know what to do! I have a vet visit scheduled for 1/6 but I want to help him be comfortable and maybe get his appetite back.

It sounds like they picked up a infection of some sort, or maybe a virus that made them both sick. It could have also been brought on by the stress of all the changes recently. As well as the stress could be putting him off food.

He could also be holding out for the good food, either wet food if he likes it or he doesn't like the new dry food and is holding out for the old stuff he used to like. (I have seen cats do this) Try all the old foods he used to eat.

Also try rotisserie chicken, its a popular food stimulate.

And try to heat up his food too, it makes it smell more appetizing.

Good luck!

Not quite sure I know exactly what your question is here.  We cannot diagnose the issue via the internet, but you've already seen two different Vets, from what I gather, and now you are going to a third, is that right?  

Sounds like someone thinks this is stress related, since you were given Feliway to try.  Cats under stress do all kinds of strange things, including not eating.  Has anyone advised you to force feed him?  It is imperative that he not go very long without food.  Is the RX food A/D?  If not, what is it and why did they prescribe an RX food?  Why did they suspect possible Megacolon and/or blockage...does he have a history of constipation?  If so, dry food is probably  not good, as cats prone to constipation need as much moisture in their system as possible and they typically don't drink enough as is. 

The vomiting on cat grass is not uncommon, so that may not have anything to do with what is wrong with him.  The raised temp could be from stress...some cats have a higher temp just from being at the Vet's office.  I'm not saying this is the case, just saying it's a possibility.  

Did the kitten get the new mixture of food?  If so, that could explain HIS (or HER) diarrhea, as a change in food, especially dry food, needs to be done slowly, over a period of about a week.

As far as getting a cat to eat, some cats are just notoriously picky.  One of mine will act like she's starving, yet I can put several different foods in front of her and she might reject them all.  As the previous poster said, maybe he's holding out for the right food.  Keep trying, and also try some other toppers, even parmesan cheese sometimes works.  But as I stated earlier, if he won't eat, you need to water down some canned food and syringe it into him, otherwise you'll have worse issues.