Great question, thanks for writing. I think I understand that you and the mum are no longer together, and that you are the father. This information is not required for the response, but it may play a role in what is going on with the boy.
First, the research on "bonding" has been rather clear - the "critical period" of the first day or two has been largely discounted. It seems that if an attachment can be made within the first week or two, the child will be fine. What may be more relevant here is the mum failing to show her softer side. In fact, the research is absolutely clear that the "comfort" side of the caregiver is what actually facilitates attachment. A fellow named Harry Harlowe did a series of experiments in the US with monkeys and found that being soft and gentle with the babies was the most important aspect of bonding.
Fast forward to today... if you and mum split up, I am guessing that there is fluctuation in the boy's life. I don't know how often you visit (if you are the father), or if he has a stable male figure in his life, but mothers and sons form the most difficult post-breakup pairing, with sons almost invariably acting out aggressively. Sprinkle in a little depression on the part of mum (very common following a split, particularly when she becomes a single parent), and you have a boy who is hurting, and a mum who may not be well fitted to be both the disciplinarian and the kinder, gentler motherly type.
My advice is twofold: (1) stabilize the boy's life as well as you can. Make visitation regular, consistent, predictable... bring a sense of calm into his awareness, particularly as parents are concerned. Too, no bad mouthing or open hostility between parents when the boy is around. Big no-no, and (2) encourage mum to open up and allow herself to feel connected to the boy. Discipline is fine, but if that is the only way he receives attention from mum, then he will always go to that method when he is feeling disconnected. Begin to reinforce the quieter moments when he happens to be acting graciously toward his mum.
And if all else fails, a good therapist can help to re-create the relationship so that both mum and son get their needs met. If you are satisfied with the answer, please hit "Accept." That is the only way I receive credit for my answer. Thanks-