It is true that many people assume that older male dogs may show serious humping behavior, particularly if they are not neutered. However, it is not uncommon for a young puppy to hump.
begin to learn to walk between the ages of 2 and 3 weeks old. Even at this newborn age, some will begin humping as well. When new owners bring home an 8 week old, that puppy may already have a habit of doing this.
As the pup grows, at an early age- usually by 4 or 5 months old for toy breeds - the dog will enter into puberty. At this time the sperm is viable and humping urges can be much stronger. Once a dog is of breeding age, he will have a strong sense of any females in heat.
In regard to what your own puppy is doing and the specific reasons for doing so, it is suggested by many animal behavior experts that this is an action that the pup takes to show dominance.
All dogs think of themselves as being in a pack. When they are little, their pack is their littermates. When they are older, their pack is normally their human family and any other animals that are in the household.
So, even at this young age, your puppy is most likely trying to show dominance over his siblings in an attempt to show that he is the leader of the pack. This is not an act of mating or trying to "tie"...
It is his way of trying to take his place as the Alpha dog. For this reason, it is normal for him to be doing this to both genders.