Symptoms, as you describe them, sound very much like hemangiosarcoma. You could be more sure of this if he'd not had access to wild/dead animals, or ingesting other things you might not be aware of, tho he did not have any usual symptoms of GI distress like vomiting or diarrhea, or pacing. Looking back, over the past month or so, did you notice any periods of low energy...what sometimes happens with hemangio is that they have small bleeds (tumor is usually associated with the spleen), which sap their energy, then the blood is resorbed until the next time. Until "the next time" is a large (internal), bleed, which takes their life. There is also a vascular bleeding disorder that has similar symptoms, the name of which escapes me at the moment.
I am so sorry you lost your boy, 10 is a common age for these issues to surface, and unfortunately is way too little time to have them with us, but is the reality of their life span. Perhaps when your heart has healed, or maybe to even help heal it, you will decide to know another one...they are all so incredibly special. Take care, welcome to the board.
mom to seraphina blue & the cashman
miss jeni-take-a-ride, rescued 7/07
shangri la's great white caesar, rescued 4/09
hearthside's cinderfella (RIP 4/20/09)
shep von bellefontaine (RIP 6/9/10)
voodoo lily (dsh) & cricket (african grey~RIP 7/13)