Get the lad as a named driver on your mum's policy and be done with it!
Board of hearing people on here talking about all this fronting bollocks recently
There is apparently a new term of 'cunting' by insurance companies - that is where they rip you off by charging thousands and making it as hard as possible to pay you out - no matter how truthfull you are.
You need to get it at a realistic price - that is less than about £1000. If no company will do that then the only realistic option is to add him as a named driver. End of.