My mum died of metastatic cancer recently
It sounds as though he needs someone to take responsibility for his overall care ie an oncologist. They seem like they are still trying to find the primary and get tissue for diagnosis but that is very invasive and not clear if it's going to be worthwhile. They did this with mum for a while too. She had an open biopsy in the end just to tell us it was the breast cancer she'd had years before which we had all guessed anyway. It did not seem to make much difference as she was too ill for chemotherapy by then anyway.
The leg pain might respond very well to local radiotherapy if it's mets. Mum had that to some painful spots and it was always great. She also had a expensive antibody injection for the he bone mets and That really helped her pain too. Both those were better than morphine which she hated taking due to constipation and confusion it caused her.
When she stopped eating low dose steroids helped but again did affect her mentally. She survived mainly on Fortisips for a long time.
Getting hooked in with the hospice was the best thing we ever did. She went to day hospital there for a long time and it helped her practically and coming to terms with it all and helped us as well. Hospices are usually charities so not quite NHS and often you can refer yourself. I would honestly ring them even now and find out what they can offer. Mum died in the hospice she knew and it was the best death she could have had even with Covid restrictions. They let us all be there in the end which the general hospital never would have.
Macmillan were good too at the end. The Macmillan nurse gave a lot of the other services eg GP a kick up the arse to get stuff done for mum like carers and a commode at home when she needed those things and helped get her into the hospice at the end.
She had a DNAR and a living will to say she did not want resuscitation which I also think is very important to get done and we spoke a lot about her wishes at the end and for her funeral which meant I could be sure what she wanted. She moved money around so that dad would be able to access it after she was gone, divided up her jewellery, wrote to all her friends warning them. It's horrible to think of this stuff but she was so brave and her doing those things made our family's life much easier after she'd gone which was exactly what she wanted. She said there weren't many things good about having terminal cancer but having notice was one she would make the most of. That and the blue badge.