The reason you didn't find anything is because there's currently no insurer which will provide temporary car insurance for a person with a Malaysian driving licence. The international driving permit is handy if he was going to get a traditional car insurance policy, that is, a policy for the duration of 1 year but for short term car insurance, it doesn't have much use here in the UK. Different countries have different laws and therefore it is more difficult to offer insurance products to people coming from abroad. People with EU driving licences have no problem because the European Union shares pretty much the same laws as the UK.
You can still go to France in the second hand car but your brother won't be covered to drive it. I'm assuming you have a full UK driving licence here.