Is there any more risk for someone with learning disabilities to fall of his bicycle?

Yes, if the SLD is. From deficits in any or all of the following neuropsychological areas: visual-motor; visual -perceptual; proprioceptive/kinesthetic (coordination of body parts with each other/awareness of where one's body is in time & space). Or these neurodevelopmental problems: poor coordination; non-fluid gross motor movements (legs); imprecise fine motor movements (hands). Or poor vision.
Perhaps. The main thing I would say here in answer to your question is that for a child with any sort of neurological condition, it is very important that he or she protect his or her head at all times while on a bike. In that regard, a helmet is absolutely necessary to prevent any head injury in a fall. For my kids, if they were on their bikes, they wore a helmet.!