he will be even more heartbroken if dont breakup with him... and the poor boy will end up not being able to trust peoples feelings bout him in the future.. he will be upset when u breakup with him,but atleast it will be something he can learn to get over, ans you guys could possibly remain friends, but if you string him along in a relationship which u dont want to be in.. it will destroy him..

and as for you mum... she needs to realise that nit's your choice not to continue a relationship with this boy. does she really want to foce an emotionless relationship onto you, does she really want you to be unhappy? its only going to cause you to resent her and to resent the boy..
your mum should also respect you privacy and not be chatting to him on ur behalf... change ur passwords..

good luck