A one year old is not expected to stay seated long as they learn by being actively engaged. The life of a one year old involves multi-sensory learning such as touching, hearing (listening) and seeing. They tend to want to be 'mobile' in order to use their new skill of walking! (And sometimes, running) I would be much more concerned if your toddler was not curious and want to wander about after a few minutes.
However, if the child is unable to sit in the seat for any amount of time, even a minute or two, then you may want to use non-verbal visual cues to help remind him to stay in place, even for a brief while.
I highly recommend checking out the links listed below.