If his gums are white, then I am very worried about Maxwell. Given his kidney status, this white discoloration is likely anemia secondary to the kidney disease (since kidneys actually produce the hormone that tells the marrow to make red blood cells). And I have to say that while I am concerned about his gut signs, to see this tells us we need him seen as soon as his vet is open (sooner if he is weak or having breathing issues) as this does need to be addressed.
Otherwise, in regards to the GI signs, while this could be related to gut infections, pancreatitis, and ingestion of harmful items; with his kidney issues we also have to be wary of progression of that disease. Especially as we often see nausea from kidney issues alongside uremic gastritis and even secondary opportunistic infections preying on his weak immune system. And I'd not that any of these would cause both his vomiting and his anorexia.
Again with this in mind and your being away, we'd want him seen. Though at least in the meantime until you do have him seen, we can try some home supportive care to at least try to settle his stomach. To start, if he hasn’t just vomited (since otherwise we’d need to rest the stomach for a few hours first), you can consider treating with an OTC pet safe antacid like Pepcid (More Info/Dose @http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid
) or Tagamet (More Info/Dose Here @ http://www.petplace.com/article/drug-library/library/over-the-counter/cimetidine-hcl-tagamet
). Whichever you choose, we’d give this 20 minutes before offering food to allow absorption. Of course, do double check with your vet if he has a known health issues or is on any medications you haven't mentioned. Also if you give this and he cannot keep it down due to nausea that is usually a red flag that we need to bypass his mouth with injectable anti-vomiting medication from his vet.
Once he is more settled, you can plan to try small meals of a light/easily digestible diet. Examples you can use are boiled chicken, boiled white fish, or scrambled eggs, or meat baby food (garlic/onion free only) There are also OTC vet diets (ie Hill’s I/D or Royal Canin’s sensitivity) that can be used too. The aim of these diets is that they will be better tolerated/absorbed by the compromised gut. Therefore, it should get more nutrients in and result in less GI upset. As long as improvement is being seen, I usually advise continuing this until the signs are settled, and then weaning slowly back to what you normally feed.
Overall, I suspect all of your lad's signs are due to the kidney disease worsening. The gut signs are a worry if he is starving himself (as they can develop secondary liver issues doing so) but the pale gums are even more a worry. So, we do want him seen as soon as we are able but you can at least try the above until he can see his vet to soothe his stomach.