Hello and welcome to HCM,
It would have been good if you had posted the genetic disorder from which your son is suffering.
Anyways, presence of large amounts of white colored frothy sputum from mouth suggests accumulation of secretions in the upper respiratory tract
and/ or upper gastrointestinal tract.
All the secretions produced by the nasal mucosa, oral mucosa
and pharyngeal mucosa are swallowed by the action of the pharyngeal muscles.
Any abnormality in the muscles, inco-ordination of the muscles, etc. can lead to accumulation of secretions in the oral cavity and inability to swallow them.
You need to consult your pediatrician
This sign could be a part of the genetic disorder that your son is suffering from.
Thanks and take care
Dr Shailja P Wahal