
Hi, am wondering, was Mr Yellow one of the lower weights at birth? If so, in my experience, he will gain at a slower rate than the "bigger" pups in the litter. I'd say as long as he's weeing/pooing, quiet and content and not squawking, then all is probably well.
Our smallest pup in the last litter (we had 2 smaller but one was struggling a bit) was helped along with nutridrops just to keep her on an even keel. She was a bit slower to gain as you've described, but wasn't sick at all, just a bit smaller. By 8 weeks, she was still a bit smaller, but perfectly healthy.
Just to put into perspective, our first litter of 10 had an average weight of 10 - 14 oz. Second litter same bitch different father, 7 pups - 5 over 1 lb and 2 x 11 oz, so not too small really, just that the other 5 were whoppers! The bigger pups were probably stronger and more vigorous than the smaller 2 girls & without any intervention on my part, they may have slipped further behind as the bigger ones were storming ahead! All healthy none the less...
Generally they'll catch up somewhat when weaning starts. In the meantime, make sure he gets prime location on the teats and doesn't get pushed off (I found holding my hands either side prevented my pup from getting pushed off).