My standard poodle just had a litter of 9. One is definitely smaller than the rest and I noticed it especially about when they were about a week old. My records show he was born the same weight as the others. But he kept getting pushed out of the way by the more aggressive eaters. Mom only has 8 teats, so there's competition. I didn't want him to give up as he appeared to be doing, so I watch carefully and always give him the hind teat and even more if he seems to want it. Some of them at 3 weeks are getting huge and these are the ones that nurse even after the milk is gone, they nurse every drop out of every teat there is, and it pays off. The "runt" doesn't want to work that hard, he leaves the teat at the first indication that the milk's run out. I am fascinated how their personalities are showing already! What I am most surprised about is how they were barking and trilling (growling?) at 2 weeks. So cute!
you probably need to supplement a bottle. He is probably not getting much milk and is weak because of it. I perfer Just Born, it is pretty easy to find. If you bottle feed him, he may come around. I would start asap.
I had a runt and she was the best (in conformation) of the entire litter, a definite champion, if I can get the time to show her! My suggestion is , take the bigger pups and put them in a shoebox and let the little guys nurse first, get the big full teats and then put the big guys in to finish up. Bottle or tube feeding is OK but nothing is better for baby than Mom's milk. And Ruffian is still a petite gal, but has the heart of a giant!