I hate to say this, but...
The likelihood of you being awarded sole parental responsibility is very, very, very low. As in, right next to non-existent low. You might get residency, but you probably won't get sole parental responsibility. It has nothing to do with mother vs. father or injustice in the system. It's just a raw fact that it's just not ordered very often.
Legally, you must prove the child is at a significant risk of harm if he remains living under their roof, and one incident that was against you, not the child, isn't going to convince the court to exclude the mother from making decisions about the child you both made.
I understand this probably isn't what you want to hear, but it's fairly common for warring parents to have an unrealistic view about court proceedings in relation to parenting matters. Things have changed a lot since the Family Law Act 1975 was amended in 2006 to focus on the best interests of the child, not the rights of the parents.
First up, you'll be fighting the presumption that the parents have shared parental responsibility. To rebut this, you need to prove that the presumption of shared parental responsibility should not be applied because it isn't in the best interests of the child. Basically, how is it bad for the mother to help you make decisions about the child's education, health care and living arrangements?
To address this properly, the court considers the provisions of Section 60CC of the Act when deciding what's in a child's best interests. There are two primary considerations, the first being the benefit to the child of having a relationship with each parent, and the second being the need to protect the child from harm caused by family violence, neglect or abuse.
The second consideration takes precedence over the first, but the court generally addresses this by making orders that protect the child from harm while still enabling the child to have a meaningful relationship with the parent, for example, by ordering supervised visits or ordering urinalysis testing.
After that, there's a lot of second considerations. How involved has each parent been in the child's upbringing? Have they sought involvement in major, long-term decisions about they child? Have they paid their child support? Have they understood the responsibility of parenting? Do they understand the child's emotional need to have a relationship with the other parent? They're all outlined in Section 60CC of the Act.
To have a case, you need evidence. A lot of evidence. Not just one assault, but many assaults, on many occasions, over a long period of time, and medical records or police records to back those claims up, and for the child to have been exposed to some or all of it. If you have very little in the way of evidence, you would be better seeking equal or substantial and significant time with equal shared parental responsibility, rather than sole parental responsibility and residency.
As well as my legal training, I also speak from personal experience on this. For many months, my partner and I were convinced his daughter's best interests were to live with us and spend time with his physically abusive ex-wife, but there is often a far cry of difference between what WE think and what the COURT sees. Even though we have been subject to extraordinary animosity at the mother's hands, at the end of the day, the child is well-loved in both households. She loves her dad and she loves her mum, and it would be a detriment to her to have her mum removed from her life solely because of lingering anger over a failed marriage and the conflict between the parents that the child never asked for.
Remember, it's not about the mother, or how terrible a parent she is. The child loves you both and has a right to spend time with both of you, so make that the focus of your case.
