Oh, I am SO very sorry.
Sadly, many of these tumors are like this...they just seem to pop up out of no where. I have had it happen twice...once with a foster and once with one of mine. You did what was best for him ... I wouldn't want a pup to have to experience the pain of surgery only to end up being in pain again. I know how hard those decisions are....sadly we have to do for them what is the most painful for us.
I hope that very soon your good memories will help to heal this pain of losing your sweet boy.