Thank you Margaret,
First, I have to say that I am quite concerned to hear that your local ER vet is not being helpful. Especially as this is quite a serious situation. Therefore, I do want to note that if this ER facility or out of hours service is not helping, then do consider checking the RCVS Register (HERE) to find an alternative local vet (since all should have their own out of hours care) or Vets Now (LINK) who are open all nights/weekends. That way you will have proper support over this weekend and a means to have his blood sugar tested and this situation addressed (since major crashes can require IV fluids with IV glucose to stabilize these kitties).
Now based on the signs he has been showing, I'd be concerned that Bruce is having a hypoglycemic event (or blood sugar crash). If he has been under the bed all day (perhaps feeling unwell due to some ongoing infectious process or GI upset that will make him less then keen to eat), then doing so and not eating as normal could have triggered this for him. Otherwise, we can see some diabetic cats suddenly go into a diabetic remission and stop requiring as much (or any) insulin.
Therefore, at the moment, we want to get him eating. If he is lethargic, sleepy, and uninterested, then do rub some glucose syrup (or honey, karo syrup, pancake syrup, jam, etc) on his gums to give him a sugar boost and then tempt him to eat. You can offer his normal food, kitten food, or anything that he is keen on (ie boiled chicken, white fish, scrambled egg, etc). We just want to get that suspect low blood sugar up. And it goes without saying that we do not want to give any more insulin until he has had a blood sugar checked (as giving insulin to a cat in a hypoglycemic state could lower the blood sugar further and lead to serious complications or be fatal).
If you try this and he isn't perking up, especially since we likely have another opportunistic issue that was making him poorly to withdraw in the first place, then we'd want to have him checked urgently (potentially at one of those alternative vets). The vet can check his blood glucose to confirm our concerns. If he is severely affected, then again IV fluids with IV glucose support is often an important treatment for these kitties. Otherwise, depending on their exam findings, the vet can treat to tackle the underlying issue that may have set him off. And in all cases, we will all want to keep an eye on what his blood sugar does as he stabilizes with everything else. Because as I noted, it is possible that this has all been triggered due to a diabetic remission and he may not need as much or any insulin after this.
Overall, based on his signs, these would be my main concerns for your lad. Of course, he has picked a terrible time to do this. Still, we want to get some sugar on his gums to give him a boost and then get him eating. And while you do that, it is worth seeking out an alternative local vet because we really need to get an idea how low that blood sugar is before we consider giving any more insulin. And if he is brewing another issue (like a stomach bug), we'd want to have that addressed to ensure we can keep him eating and avoid any more crashes.
Please take care,