All the pupppies are
Growing one is not he is to small and weak to eat from mom need help

Hand raising something so small is difficult, but not impossible, and when all goes well, very rewarding.
Make sure to get a puppy milk replacer, not cows milk.
You can find puppy formula and any pet supply store or even Wal-Mart.
Follow the directions on the can or bottles for how much and how often to feed according to his size.
If you have a small kitchen scale you can weigh him on that for accuracy.
Puppies can aspirate easily, or suck milk into their lungs causing a serious pnuemonia, most do not recover from this.
Be very careful regarding the nurser you use etc, good idea to get an associate at the pet supply to help with choosing the feeding supplies and how large, or in your case, how small to make the hole in the nipple.
And make sure that when you feed the pup is belly down in a natural position.
You may have to adjust according to what he will take at one time.
Keep some glucose substance on hand, puppies that small can become hypoglycemic very quickly.
You may be able to find that in the pet supply store also, if not Karo syrup will be fine, rub a little on the gums and under the tongue if he becomes limp or non responsive.
If mom is cleaning him and stimulating him to pee and poop then let her,
Otherwise you will need to do this also, shortly after every feeding.
With a warm damp cloth gently rub the genitals and anal area mimicking
What mom would do.
Make sure he stays warm, he cannot at this point regulate his own body temperature, if mom is not keeping him warm you can keep him on a towel covered heating pad, not hot, just cozy.
These are the basics, but keep in mind, that sometimes it is not mother natures intent for every pup in every litter to thrive, as heartbreaking as that can be.
Hand raising is difficult, but if you don't discourage easily then you will do fine.
In any event it may be worth a trip to the vet just to make sure that there are no obvious defects or problems.
Sorry to have gotten so long-winded, just trying to cover the bases.
Best of luck with your little one.