Hi,
To check whether your son's DLA makes him eligible for an S1, you need to contact the Exportability team:
https://www.gov.uk/claim-benefits-abroad/disability-benefits
Then your son needs to apply to the Overseas Healthcare Services team for his S1. That team has recently transitioned from the Department for Work and Pensions to the NHS Business Services Authority. It would probably make sense to email them in the first instance:
Kind regards
Kim