It is tough - I had been watching CJ for quite a while and wondering how he was doing.
I think you'll know when the time comes - your son will understand - it would probably be good if he was there at the time (that is assuming you have to go to the vets) Apparently some pets just die in their sleep
I learned something new:
We could have taken his body with us to bury, but the law in my city is you are not allowed to do that - we would have had to take him outside the city limits i.e. if perhaps we had a cabin or a summer home
We just paid extra for the cremation instead
If it's a small breed he probably has quite a few good years to go - so don't worry