Sudden, jerky, shock like movement in a child during sleep is called sleep myoclonus
and is usually harmless. Sleep myoclonus term is replaced now a days with periodic limb movement disorder
and it should not be confused with restless leg syndrome
about which you have read on the internet.
Restless leg syndrome is more common in elder age group and is more frequent in nature ( more than five per hour). Restless leg syndrome has no relation to sleep and it occurs in awake condition also.
So if this movement is not frequent or persisting for longer duration and not associated with sleep disturbance then you need not to worry.
Your other concern is about his allergy. So get an serum IgE level done if episode is frequent. If it is elevated then long term use of mast cell
stabiliser like montelukast
will give some relief.