Condolences Alex. I understand.
The hardest thing I have had to go through thus far in life was the death of our first sheltie, Kotie Sue. We had to put her down on April 8, 2007. She struggled a year with bladder cancer. We did everything we could and bore all expense to do what we could, but the cancer only briefly went into remission. The hard thing about this cancer is that it is tumors that grow between the lining and wall of the bladder, so a dog will have no symptoms except for more frequent evacuation until the tumors block the urethra and cause uraemic poisoning. It is hard to reconcile that your loved pet is herself only with death being imminent. There is the issue of one's own selfishness to deal with in trying to maintain the pet's life. The vet assured us that this becomes the case only when it is evident the dog is ready for death and its owners continue to fight for its life.
In Kotie's case we knew when she was ready. Kotie hated being wet, and one day she just laid in the yard during a thunder storm getting soaked.
It is ironic that as she aged, I found myself withdrawing from her emotionally little by little trying to distance myself from the pain of her ultimate demise. I did not like this at all but it wasn't something I could control. When her cancer was diagnosed and her death became a reality, I no longer distanced myself but became free to love and enjoy her again.
I was broken up for half a year easily.
I had started looking of for another sheltie pup about six months before we put Kotie down. On April 1, I brought home Brooke, an eight month-old show dog, whose owner decided that her head was too small for the standard. My wife's birthday was the next day. Kotie took to Brooke right away, and her presence helped Kotie during her final week.
Brooke helped us through it by giving us a new target for the affection we had for Kotie. This is a great opportunity to teach your son about love, grief and loss. Try and bring a new object of that affection into the family as quick as you can. The new dog won't replace the Mastiff, but will bring new joy on its own.