Dad beat cancer twice but scared of a 3rd time

Hi All,  not sure where I am going with this but the last decade has been tough on my family and I am worried about my dad's mental health and my mums health as his carer. Dad had prostate and bladder cancer a few years ago now and had a couple of doses of chemo which they had to stop due to damaging organs so had radiation instead. This has had a severe impact on his 'innards'  and  has to watch what he eats,  has lots of problems etc,  but the cancer had gone.  On top of this though he has a heart condition which he takes lots of medication for,  he also has diverticulitis which flares up and he has charcot foot which is horrendous and his feet bones constantly fracture and make his feet deformed and leads to ulcers and bones coming through skin. He can barely walk properly. He also has emphasymia and now asthma. He is 82. Mum is not too healthy either and is 76. She is exhausted with years of stress,  worry,  hospital appointments,  dad's mood swings,  constantly on the go,  no let up for years. Dad never saw a professional to talk to after his cancer treatment. We think he should have. He had a lot of nightmares,  became very emotional and has changed a lot. He does not stop talking about 'when I had cancer...... " at every opportunity he has. As if he was the only person that ever had it. This is not meant to read as me not understanding,  its more explaining our situation. He has been clear of cancer for a few years now but constantly worried it has come back. I can understand this as I had a scare and major surgery 3 years ago. On Friday he has appt with urology as he not weeing right.  He has to self catheter every fortnight due to scar tissue build up. He thinks they will say the cancer has come back. He has massive irrational arguments with mum to the point she thinks he is going mad. He doesn't seem to worry what he says to people,  how he may upset them. He comes across as selfish and, attention seeking. We are finding it so hard. We may not have his symptoms but we have been with him every step of the way trying our best. Constantly doing things to help,  suggesting things,  buying things, we used to take him out every day we could,  do nice things. Now I have moved away and dad's health is so up and down its hard to plan anything. Every hour can change regards how he feels. Now he has told my mum after 54 years of marriage he wants them to live separately in the house!  And that she doesn't love him they have no interest together,  which is ALL nonsense and he has been repeatedly saying for months,  he doesn't know why they got married as they have nothing in common. He flares up at the slightest thing and we all walk on eggshells just to try and keep the peace for mums sake. My brother opted out years ago. He can't really stand dad anymore and only does the necessary to keep in touch. I have always said to dad that I think he should talk to someone,  but he knows best and no one can tell him anything. Although he said the other night he unburdened himself talking to the moon,  fair enough,  but it's one sided!. I am really worried my mum will have a breakdown or stroke. Can I convince my dad to seek help???.   He does not use computers,  he cannot travel far. He likes to watch TV a lot or in his greenhouse. He has almost no attention span for anything else. All he wants to do is what he can't really do. He, always wants to go for a coffee and wants to see the sea. He feels trapped stuck inside and gets angry that mum is quite happy not to go out all the time and look on computer or do jigsaw or just relax.  Dad cannot relax and he is constantly fiddling with his hands,  wringing them,  scratching things. I think he really needs help. Can anyone suggest what we could do please.