It sounds like that he needs a bit of socializing with other people and other dogs to get the bonding of him and your mum reduced.
It is always nice if a dog bonds with a person but as you have found out it can have dire consequences.

Is there anyway that you can be around the dog without your mum being around as well say for a day or two, this way the dog will only associate itself to you, it will pine of course but it will learn. You are going to have to be strong and your mum is also going to have to be strong and a bit negative in her attitude and behaviuor towards the dog but you must also show a positive masterful attitude. Do not let the dog see that you are afraid or weary as it will increase its dominance of you.


I hope this helps