How very sad and traumatic for you.
When my Dad was diagnosed with bowel cancer back in 1999, I was so totally shocked and in denial that I too "went off the rails" somewhat. I started going out all the time and drinking excessively. I was traumatised within, and yet was unable to discuss my dads cancer with him. His cancer had spread to his liver and was in his blood, but I could not talk to him about it, despite us being the closest Father and daughter there was.
It was only when he was so desperately ill towards the end and was diagnosed with 8 weeks to live, that I finally faced everthing, and spent every single day and night from thereon at his side. I did all I could to make up for the time I had spent avoiding him, and was with him, holding his hand as he took his last breath.
To this day, I am ridden with guilt that I did not do more for him and did not face his ordeal with him during his final year of life. Our closeness was in my mind, the reason for my distance at this critical time, and I can therefore, somewhat appreciate what your son is going through - he is simply so distraught and traumatised by what is happening to his precious Mum that he simply cannot face the facts.
He needs counselling, and support. Are there no family members that can help him through this? You both need as much support and help that you can get - I truly feel for you all - and wish you the very best of luck with your ongoing fight