Thanks so much for the additional information.
Since he urinated so soon ago, we can take a blockage off the list of possible causes for his behavior which is a very good thing! The urinating outside of the box, however, could indicate something going on with his bladder although it might reflect the stress of having to interact with your cats, too.
Normal pale pink gum color is good as is the fact that he doesn't have any abdominal pain.
It doesn't sound to me like stress would necessarily cause his behavior since he's already been with you for two weeks now. If he were going to be significantly stressed to the point of being withdrawn, I'd have expected that you'd have seen this behavior before now...unless you've been keeping him isolated.
His lip smacking could be secondary to nausea (even though he might not be actively vomiting) or possibly pain or something going on in his mouth. It's not specific to one particular condition, in other words.
1. It would be helpful if you could take his temperature just to confirm that it's not elevated (although this isn't always the easiest thing to do in a cat); normal is between 100 and 102.5 F. If it is high, then this would explain why he's more withdrawn but it might not necessarily explain his lip smacking.
Of course, then we'd have to wonder why the temperature is elevated. Explanations for this could be: a bacterial infection, a virus , or toxoplasmosis.
There is a condition in cats that is called Fever of Unknown Origin--this is exactly what it sounds like which is that we can't find an explanation for the elevated temperature. Treatment is primarily supportive which consists of fluids and drugs to reduce the fever. Most cats recover very quickly.
2. Pancreatitis can happen very suddenly and might explain his behavior. We don't have a good explanation for why this condition develops either but he's about the right age for it. About 30% of these cats will also feel a little nauseous.
We do have a test for this (spec fPL) although I've come to doubt its reliability in some cases; treatment consists of fluids and pain medication (we believe the pain is why they stop eating).
3. One other possibility that I'll mention just to be complete might be some sort of systemic disease. I've seen more than a few cats where stress (such as a new living situation) has triggered an underlying condition which they've managed to mask or control. If he were much older, I'd be more concerned about this possibility.
4. I've also seen a few cats who just have "off" days but will return to normal, usually within 24-48 hours.
One over the counter drug which you might consider giving him for nausea would be Famotidine although it may be difficult to find in the UK. The dose would be 1/4th of a 10 mg tablet twice a day. Another option would be Omeprazole at a dose of 1/2 of a 10 mg tablet once a day.
Maintaining hydration with water or Pedialyte is important, especially if there’s a fever. Since he's still eating, you can mix canned food with a lot of water or tuna juice and see if you can get him to "drink" his meals.
I would dearly love to be able to tell you what specifically might be the cause of his behavior but I hope this helps give you some possible explanations.
Hopefully, he'll improve if you give him something for his nausea and he will start interacting with the family again within a short period of time. Deb