Considering there is no court order in place, you can pick up your child and take them into your care as you have PR. However, this would really apply to things like collecting him from school etc. You can't walk into someone's house to get your child as you would then be breaking other laws which would not go well for you if this went to court. You also need to think about the impact upon your child if you did that. Would it be nice for him to see his mum shouting and screaming, trying to grab him and take him back? It would be hugely traumatic for him.
Having been through this multiple times myself with my husband and his son, I would suggest that you obtain advice from a solicitor. If you can't afford a solicitor, there are numerous books to guide you through the process, such as "The Family Court without a Lawyer" by barrister Lucy Reed.
Before making an application to court, you would need to attend a Mediation Information and Assessment Meeting (MIAM) with a qualified and registered mediator. You don't have to actually engage in mediation but it would go in your favour if you attempted it as the courts usually direct this to happen anyway. If mediation breaks down or you don't wish to take part, they sign a form and then you can apply to court. There are some exemptions where you don't have to go through this stage though.
You can apply to court without a solicitor on form C100 and if you have low income, you can also apply for fee remission on form EX360. You would likely be looking at around 6 weeks until the first hearing which would only be a directions type hearing if there is no agreement.
I would advise to do everything by the book as the last thing the courts want to see if parents putting the kids in the middle of these conflicts.