Contact your Local Authority and make enquiries, (schools should refer a child after 15 days of medically related absence but often they do not do so, especially if the absence is related to mental health needs).
Explore the options in your local area for alternative schools and learning environments. You may be able to change schools if you can find one that is more suitable or willing to support your child and work with their anxiety.
SENDIASS may be able to help you identify a local setting or help you to access it through an EHCP.
Some people manage to arrange funding for EOTAS (Education Other Than At School) which includes hospital school, online schooling or home tuition.
Your Local Authority will have a policy and details of local provision, you can find out more through this link:
https://www.gov.uk/education-attendance-council