Hi Lizzie and welcome to Talking Point.
You've raised a very difficult subject, and one which probably has no answers only more questions. Caring for a spouse with dementia is a uniquely challenging operation. Over the years, most husbands and wives become "units" and tend to resist outside influences. So when one of them falls ill the other may do their very best to keep "outsiders" out, particularly if the illness is something like dementia. The well spouse may try everything to to protect the ill spouse, even when such protection (such as refusal of outside help, or refusal to push such issues as regular washing etc) is for the spouses benefit.
In other words this counter-productive behaviour may be coming out of love, rather than neglect. Which is not to say that you shouldn't try and get through this barrier, but you do need to recognize where it's coming from. You say there are children - what do they have to say? Also, and this may, probably isn't, the case, but you should bear in mind that it's not unheard of for both spouses to have dementia, so it's possible he isn't rational. Mind you, having had my own experiences with hospitals I'm can't say that it is always a bad idea to discharge someone against consultants advice - hospitals aren't on the whole great places for dementia sufferers.
What I'm trying to say is - if you approach him in an accusatory frame of mind he will almost certainly reject any help. He might reject it anyway if he feels that you are judging him. The fact of the matter though is that no one can be expected to takeover the long term care of a dementia sufferer without outside help, even if that help is just to occasionally give them a spell of respite so they can take a break.
It will depend on family dynamics - some people will only except help from family members, while some will only allow "professionals" to provide that help. Someone is going to have to help your uncle find the support that he can live with, otherwise they're both going to end up in hospital.
I'm moving your thread to the main support area because I think you'll get more responses there.