The most common income replacement benefit for people who are ill or disabled and unable to work, is Employment and Support and Allowance (ESA). More information about ESA including eligibility and how to claim can be found on the government website. https://www.gov.uk/employment-support-allowance/overview
Does your son receive Disability Living Allowance (DLA) or Personal Independence Payment (PIP)?
If in the future he receives an income related benefit such as IR ESA then yes he would qualify for free prescriptions. You do not automatically qualify for free prescriptions if you are on DLA, you have to be on the relevant benefits. More information about entitlement to free NHS prescriptions can be found by following this link http://www.nhs.uk/nhsengland/healthcosts/pages/prescriptioncosts.aspx You will also find information here for other forms of help for health costs including Pre-Payment Certificates.
Here is some information about free dental care and who qualifies for this http://www.nhs.uk/chq/Pages/1786.aspx?CategoryID=74
I hope this information is helpful. Please do get back to me if you need anything else.