So sorry for your loss. He was such a beautiful boy.
I lost my 1st boxer almost a year ago to internal bleeding as well and I know exactly the look that your boy gave you, as mine gave me the same thing at the vets office. It's the hardest thing to let them go and even harder to watch as they go quietly. oh the tears are definitely coming out now. :(. Just know that you did the right thing for Bertie. It was hard to wrap my head around the fact that I put my baby down, but I found peace in knowing that he was ready even though I wasn't and he made the decision for me. It does get easier with time, but even almost a year later and I will still weep like it happened yesterday. I found comfort in the things that my Buster liked..He had a blanket that he slept on everyday so I slept with that for the longest time cuddling it like it was him. And look at LOTS and LOTS of pictures, even though you'll cry through some of them, you'll surely laugh and smile by the time you get to the end, I promise.
Keeping you in my thoughts and prayers.