UC and loss of Council Tax support

Since I wrote a previous message some time ago about my son's loss of full Council Tax support when he had managed migration to Universal Credit, I see several other posts from people losing their full Council Tax support too, though I think the reason for my son's loss of it is different, nothing to do with Transitional Protection.

As I am my son's Appointee I asked the Council Tax department for the reason for why my son has to pay some Council Tax even though he is getting exactly the same amount of money under Universal Credit as he was getting under ESA and Housing Benefit:- my son lost the full Council Tax support he was getting because he does 4 hours per week Supported Permitted Work which is therapeutic work. When he was getting ESA, the income he got from Supported Permitted Work was disregarded by Council Tax. Universal Credit display the Supported Permitted Work as simply "Earnings" and so Elmbridge Borough Council count that as 'excess income' and charge my son 20% of that as Council Tax.