Fences are usually no obstacle to cats. So, if you are concerned (I would be) here is what I suggest. 1) Let him be outdoors on a harness and long leash (obviously being monitored by you or your Mum).

2) get him one of those catio things. A large, secure, enclosed device that you can allow your cat to be in when they are outside.

Make sure that your Mum fully understands that even a fleeting second of inattention could lead to George scaling the fence/wall and escaping. The leash idea puts paid to that.