Am so sorry to hear all of this Quootiepie. I can only imagine how sickened you must feel, not knowing what's happening, not knowing if you're on your own, not knowing how he feels about you, feeling betrayed, abandonned, and hated by his mother, who sounds like a bitch BTW .... what sort of grandmother must she be to continually encourage her son to leave his small son ?!
Thing is, like everyone else has said, YOU have got to take control of this situation. Yes, that's scary - as are most things unknown - but YOU and YOU alone will be responsible for your future .... as opposed to being dependent upon someone who has let you down again and again, who has let his son down again and again.
I know that when a relationship goes wrong, it's natural to want the other person back. But you must know deep down that the person you want back ISN'T the person he really is. The person you want back is the person you hoped he would be, or the person you obviously once thought he was .... a loving, responsible, loyal father and partner. And he ISN'T being loving, responsible or loyal - to either you, or his son.
If he comes back, even if you don't have to beg him to do so, what will happen next ? You have already described lying there listening out for him, wondering what his next move will be ? And do you honestly believe that his revolting, interfering mother would leave things alone ? Could you really live like that long-term, becoming more and more anxious, with all the health implications of being mentally tortured like this ....... which then of course, could have further implications upon how well you feel able to care for your little boy .... who is bound to pick up on your unhappiness and anxiety.
Becoming independent is possible. Millions do it - there is no reason why you can't too. With each new thing you learn, your confidence will be boosted, and this can only be good for your son. There are loads of people here who can help - either literally, or by offering advice and suggestions - if you don't know how to tackle x,y or z. All you have to do is ask, and no-one will feel you are "asking too much" or "putting yourself" on them.
Please, please put yourself and your little boy first and take steps towards making your lives the way you want them to be. You will feel so much better in yourself by taking control and refusing to allow this man to mess with your head any longer. I know that right now, you may well feel dreadfully sad and disappointed that your relationship hasn't turned out the way you hoped it would, but you're so young and you have years ahead of you in which to enjoy life as it should be. That includes the sort of partner you deserve at some stage.
Don't waste any more time trying to get inside his head, analysing why he's behaving like this. TBH, at the moment, it doesn't really matter WHY he's like this ..... the fact is, he is, and you need to counter that by doing what you can so his behaviour doesn't affect you.
YOU and DS are important here, no-one else. He's obviously got problems regarding commitment and responsibility, let alone having what seems like quite a sadistic streak TBH (to be able to do this to you) and those are HIS problems to sort out. Chances are he won't, chances are he doesn't even accept he's being so grossly selfish - especially if his mother is dripping poison in his ear and encouraging him. But who cares about that ?
Believe me, gaining your independence will feel great and you can do it. Don't let him mess you about any longer. Would your mum be able to lend or gift you the money to change the locks for example ?
Take care - and keep coming back here if you find yourself wavering.