Council Tax and Care leaver Exemption

Hi. I am advising a 19 year old who has been in an out of foster care since being a baby. His Grandmother finally took residency of him and took on Parental Responsibility. He now has a tenancy in his own right and is claiming UC - since March 2020 - due to the Covid crisis the landlord failed to let the council know of his taking up the tenancy -  as did the tenant -consequently he could not make a claim for CTR and received no council tax bills to trigger him taking action. The council are now charging him council tax back to his tenancy start date in March 2020. I am seeking an exemption on the grounds of him being a Care Leaver - however I find different definitions of “Care Leaver”; the council is in Wales. Does anyone have any guidance regarding this exemption ? I can find a Consultation Document from the Welsh Government but there is no definition on the individual Council’s website regarding eligibility criteria to the exemption

I would be grateful for any advice as this is a debt this young man cannot afford to pay.

Thanks

As an alternative, maybe you could use the discretionary reduction option from 13A of the Local Government Finance Act 1992?