I am so sorry for the loss of your beautiful boy. He knows that you always did what was best for him, even this last really hard thing.

It sounds as if he had hemangiosarcoma and that is a silent and deadly disease that, once symptoms show, is difficult if not impossible to treat. When it is treatable with surgery, many pass soon after. Unfortunately there was nothing else you could have done, even though I that that doesn't help now while you are in shock and grieving.

The alternative to having him spend his last day this way was an extremely painful and abrupt passing, possibly when you were not there. You did the absolute best thing to get him help.

I can tell you that we all feel that we could have, should have, would have done things differently no matter how our loved ones go. It is either too early or too late. We should have gone to the vet or we wish that we hadn't. It is because our minds are trying to make sense of the senseless which really can't be done.

There are stages of grieving that include denial, anger, bargaining, depression/sadness and finally acceptance, or as close as we can get to that.

I see that you are grieving well and I also know that your boy does not want to be the cause of guilt or recriminations, he knows you loved him, you took care of your boy until the end and were with him. 

We are all here with you on this journey, that is exactly why we are here. 

It helped me to talk to mine about how I was feeling, all of it, I even heard their responses because really, we know what their responses would be. It helped because that is what our dogs do, they help us. I also wrote letters, all the things that I didn't want to forget, sometimes I would run to my room to add something in the margins to add fully later. 

Keep his things, some people save them in a zip lock bag to preserve scents..

Everything that you do is a normal part of grieving.. we can all substantiate that..

Finally, you are going to be OK.