That's just awful & must be such a shock for you all. I'm sorry to hear you're going through such turmoil. 
My dad was recently diagnosed with secondary liver / lung cancer from his prostate . After intense chemo that finished in June, he had mri / CT etc and we were told his cancer had responded v well to treatment. He's had monthly checkups / bloods  due to bone treatment. All seemed well. In September he gradually started getting weaker & losing weight. He went back for MRI/ CT etc in October only to be told it's spread to liver / lungs. Despite intense chemo & regular checks this horrible disease took hold of him. I believe when it's advanced it can progress very quickly. 
How often was your dad getting scans? My dad has his in in June ( clear ) and by October it was advanced in lung / liver and other than steroids he's on no treatment. 
I wish you the best during this difficult time . 

Thanks for the reply threescompany,

Just speaking to someone who has went through the same journey helps.
 

We went to everyone he'd been sent appointments to, I just feel that he should of been a high risk patient having had 2 primary cancers in 2 years. 

3 weeks ago they said it was inoperable but we could start chemo but he got weak so fast that he wasn't strong enough for treatment. 7 days have now passed in this hospital and I've been here every day, my mum and I were both staying initially but now we are taking turns but we're both getting exhausted. I don't want to leave him alone though.