According to the internet:
Autistic children often have unusual posture, including sitting hunched or slumped over. And some children on the autism spectrum will walk completely on their toes and/or walk with an unusual gait.
On a more technical note, another article describes it as follows:
Postural control impairments track from infancy to childhood and even later years. Children with autism spectrum disorder (ASD) showed a decreased static and dynamic postural stability, functional balance, and motor performance. They reported a higher postural instability and increased weight distribution in children with ASD (16-18). Although there is a regular improvement in postural control of typically developed (TD) children from age five; ASD investigations demonstrate a different pattern. Postural control does not begin to improve untill the age of 12 years and never achieves the adult level in children with ASD. Another sign of postural immaturity in children with ASD is an abnormal asymmetry in the direction of instability. Children with ASD show higher postural instability in medio-lateral compared to anteroposterior direction; whereas, TD peers show a reverse pattern. While this pattern in ASD looks like the primitive pattern of postural control in toddlers, authors explain that an asynchrony in postural stabilizing muscles may be responsible for this unusual development of postural control in ASD.