OP, £660 a month just on gas usage and standing charges not including any debt is a crazy amount of gas on a normal domestic tariff.
If as you say your bill is correct then the house must either be at sauna like temperatures or incredibly poorly insulated and drafty or you've got your windows wide open.
I would get a couple of thermometers and check the actual temperature in the rooms when he says he is cold. Pitch it as an experiment to check the heating/bills and see how you can make the house warmer.
Central heating thermostats aren't representative of the temperature in the whole house, if it is in a particularly warm or cold area it can be pretty misleading. You may also find some rooms are warmer than others. He probably isn't being unreasonable to want to work in a temperature in the early 20s as sitting still does make you cold. Over 25 is excessive and environmentally irresponsible though.
It isn't unreasonable to expect him to pop a jumper/fleece and a pair of slippers on. But it is a bit unreasonable to expect him to need hat/gloves/blanket if the family can afford to have the heating a bit higher.
Consider moving him to a warmer room and you using a cooler one if possible. I am much warmer since I started working in a bedroom rather than the lounge and if I get too cold it is a smaller space to heat.
Adjust the radiators in each room so that when the heating is on you aren't heating rooms that don't need it. Consider getting new radiators with thermostatic controls for each room. Also consider a programmable thermostat so you can control the heat throughout the day and ensure the heating is higher at times when he feels most cold and then drop it down at times he won't notice.
Start draft proofing and insulating. There are lots of free/cheap solutions that you can get going with immediately, and for more expensive insulation measures there are grants although it is worth spending money to save money and energy.
Make sure your in home display for your smart meter is visible and he can see how much he is spending per day. You can set a budget on some so that it shows amber if you are getting close to budget and red if you exceed. Agree with him what he is happy to spend on heating and set the budget accordingly them show him when it goes red. He needs to understand what turning the heating up means for family finances.