I just want my son back please help me

I gave birth at 18 after a very violent relationship I tried to do it on my own and my mental health was undiagnosed, he went to live with my mum and her AWFUL boyfriend when he was 3. I fully believe the man my mum is dating got with her to manipulate her to get social involved so he could have my baby. Before my mum met him she would of never of just took my son and not me and now I'm terrified this man is been manipulative to my now 9 year old son he tells him I dont love him and don't care about him luckily my son knows I dote on him and I spend every opportunity telling him so. I have to do this through roblox because despite a court order saying contact should be weekly she tried saying once a month is better. My mum shouted at me for telling my son I love him saying that it demeans there relationship. I am In a good place with an 18month old daughter I do not work but my mental health is controlled i am so scared my son is been emotionally abused at least my younger sister moved out when my mums partner moved in because he used to walk Into her bedroom bathroom ect uninvited when she was only 16. My mum chose him over her daughter. This man is a Councillor and has literally destroyed my mums relationship with every family member she has we all used to be so close she cant even lend me or my sister money without saying dont tell matthew. At the very least I think this guy is emotionally abusing my son if not worse but I have no proof. I need some advice they have an SGO on him but he would definitely be better with me and I know my son wants that too please help