Sounds like a hemangiosarcoma. My mother lost her German shepherd in a similar way. It really is heartbreaking.
I'm so sorry for your loss. You really did do everything that you could. Letting him go peacefully is such a hard decision sometimes... We want to try making things last just a bit longer. You put aside that selfishness and gave Moxie the peace he needed.
Make sure Tyler and Sam get some extra love. They're going to be pretty anxious.