I would most definitely NOT take this baby, stay on the waiting list for a healthy baby.
Years back I went through what you're going through now with a grieving fur baby. I then had a pack of 3 females, all 3 years apart. The two older ones passed 6 months apart leaving the baby as the only fur baby. She was depressed, her lil clown personality was gone. I gave her 6 months to adjust to being an only fur child.
7 months later there were so changes so my quest started to find her a BFF.
I found the perfect buddy, my girl was "the Diva" made that way from her two older sisters who babied and spoiled her, she was a very bossy lil gal.
My concern was, she was always the baby, could she now be a momma. The puppy I found was a big girl at 8 weeks old, I knew this puppy would not be bossed around. I took her home, set her on the floor, when my grieving girl saw this puppy she ran to her like she was a long lost pal. It was love at first sight. She bonded with the puppy immediately, my lil clown was back and took over the role of the momma.
So I do know what your Boxer is going through and his pain is your pain. But please do not rush into getting a sickly baby, as hard as it is, please wait for a healthy one and not a "so called" teacup (((hugs)))