Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today.
I am so sorry to hear that you had had to let your lad go. Still, if he had bleeding masses on both the spleen and the liver; you made the absolute right choice for him. Often these will tumors will lie latent and we won't even know they are present. The only way to diagnose them would be if he had happened to have an xray or ultrasound prior to their bleeding. Bloods wouldn't have given your vet any hint they were there.
In any case, they don't tend to be appreciable until they reach that critical stage and start bleeding. When they do, they will cause sudden deterioration and the situation you have reported (I have even had patients with this present having died on the car ride to us --it happens that quick). And while we can sometimes remove the spleen in these cases, with the tumors also on the liver his prognosis was poor even if we did do that. Especially since the liver masses were bleeding and the spread raises worries that it may have also been in the lungs.
Overall, I know this will be shocking and devastating since this type of cancer gives no warning at all. Still, you made the right choice, even if it was the hardest one to make. So, pleas do take comfort in knowing you kept him from suffering and that until that point where they started bleeding he didn't suffer and was able to live his life fully until then with you.
Please take care, my thoughts are with you in this difficult time,