HELP PLEASE

Sorry it is a long one… please read as I really need some advice! My father in law was diagnosed with PD 6 years ago and very quickly (within weeks) was on medication.
All went well and then he became psychotic at home (believing his wife was poisoning him etc) and subsequently being sectioned under the mental health act. He returned home where about a year later he got a sickness bug and taken to hospital where he received some anti sickness medication and this, again, resulted in an imbalance and another build up to a psychotic episode and sectioned once again. This time He also received 18 sessions of ECT.
He is now currently in a nursing home at the age of 65. He is refusing to see any sort of professional, family members or doctors.
He often complains that he is being ‘poisoned’ by the food and that this gets stuck in his oesophageal tube and stomach… despite many scans and tests there are no blockages or medical issues.
He refuses to speak to my husband and barely speaks to anyone at the home as he is often rude to them so I imagine that they don’t give him much attention.
We have made contact with Kings college in London who offered to see him but he simply refuses to travel anywhere to see anyone! Whilst he is of ‘sound mind’ he occasionally says that people are out to get him and he simply does not trust anyone.
I am personally struggling to come to terms with the fact that this could be his life until the end of his days… We just feel so useless. He has been given high calorie drinks as his weight is so low but is now hiding these around the room.
I would love to hear if anyone is going though/has been through similar and see if anyone had any advice.
I just cannot believe that there is nothing out there that could be done!
Thanks for reading and I look forward to hearing from anyone that made it to the end of my post.
Many thanks
Emma