I think as your waiting to move you just have to keep the dog separate from your mum until then, dogs unfortunately can pick up on certain illness and it can spark aggression sad situation though.
Most of what others have said. Work on calmness, keep him separate from your mum, speak to a professional who can help you get through this time. Castration may not be the answer depending on how confident he is, it certainly wouldn't be a magic cure.
Collies are very good at picking up on people and he wouldn't be the first to react badly to somebody if he isn't sure how predictable their behaviour is, if your mum's acting differently in his perception then he may actually be scared of her sometimes, more so if she appears her normal self to him and then something changes. It could be scent, actions, even just a way of moving; a minute change that you can't spot but is causing him to respond in a way you don't want. He's telling you he isn't comfortable so you have the chance to make changes to protect both of them.
Taking him to work sounds a good starting point, or even doggy daycare, and when you're at home act as you would if a small child was in the house, never leave them in a situation where either of them could feel vulnerable or unsafe.
So far so good He's had a good run at lunch and is now sparko on the floor by my desk. I've also spoken to a dog behaviorist today who can maybe help me brush up on my skills / provide him with a bit more stimulation long term.