Sel Private lets cover the cost of maintenance and admin plus a markup as an income for the owner. Local authority lets do not aim for a mark-up.
If he is covering the costs of his council house, then he may be getting at cost price without adding markup and profit, but that is not the same as being subsidised.
You may argue that councils have a duty to get the extra as income, that would be fair enough, and the capital cost of housebuilding may not be allowed for in the sums (I don't know whether it is or not) but if what he pays covers the cost of what he gets, that is not strictly speaking subsidised.
For you to be subsidising him, money has to go from you toward making uo the difference between the cost of what he gets and what he pays for it. People on housing benefit are subsidised, people paying full rent are not.
We went round this whole circle about council rents being subsidised a few months ago.