Pups have to learn bite inhibition, which is normally completed by 18 weeks of age. They must learn how much pressure to bite with so that they do not injure other dogs when playing. Unfortunately for humans the dogs skin is thicker than humans, especially around the neck and head where pups tend to bite mother and other dogs . Therefore the pup can learn to bite harder without hurting the other pup / adult.
Bite inhibition is taught by the reaction of the other animal (it yelps and /or stops playing with the offender). Most humans will tolerate the pup "biting" clothes or shoes or trouser legs and will only react when the teeth are closed on the skin. To the dog this means that you will tolerate a strong bite since you do not react when your outer skin (ie clothes are bitten).
If this is the case then you must yelp like an injured pup and then cease to interact with your dog for 60 seconds. This exclusion will punish what is a social animal - do not attempt to put out the pup as you will only reward him by chasing after him. Do not dance around or move the bitten limb as this will encourage the dog and reward him for biting (The reward is you giving the dog attention or moving).
Your dog may have had several months of this behaviour and if the above is unsuccessful then you will probably require a professional consultation in order to solve these and related issues.
If your puppy is insured then your policy will cover such behavioural work once you have been referred by your vet. Biting problems do not cure themselves and more often get worse so it is important to deal with the problem.