There are two seperate but related issues:
1: Access to your son, which is your right.
2: Maintenance payments, which is your duty.
Your Ex can not prevent you from seeing your child and you can take her to custody court. As for the maintenance payments, you can use this in court to show you are contributing to your son's livelihood. Your Ex has no leg to stand on, assuming the info you gave is accurate and complete. It seems she is using your son as leverage to get money from you and not for the benefit of the kid necessarily. This will not help her case should this go to court. The most important thing to remember is that the welfare of the kid is the primary concern and that is the view the court will take as well.
There are a few comments on this blog that are very ill informed. The mother can not choose on her own whether the father can be involved in the child's upbringing or not. All decision pertaining to the kid must be made by both parents unless a court has ruled otherwise for some or other reason.
The comment by someone who replied under "guest" that the UK or US systems work any other way is complete wrong. In fact, in South Africa you have a much better chance of getting a fair ruling if the spirit of our Constitution is adhered to. Contribution by the father does not have to be 50/50. Circumstances should dictate what is feasible and there is no reason why the father can not provide more. What you can do is to insist that you accompany them when any payments are made to ensure that the money goes to the child, not the mother. For instance if school or creche fees are paid, do that yourself.
The suggestion that you write your Ex a letter is a good one but get advise on what to put in the letter and how to word it. Alway stick to the two principles that the welfare of the child is the primary concern and that you have a right as parent to have access to the child.
Do not stop paying maintenance and keep record of all your contributions. She should sign a receipt or you pay her electronically to have record of the transaction. File everything and, if necessary, take it to court should that be necessary.
I do not suggest you just show up at your Ex's house without notification. She also has rights when it comes to her personal space and place of residence and you can not violate that in this case. It will compromise your case should a fight break out. Never ever consider violence as a resolution even if someone else starts it. There is no way that this will be considered as a good environment for the child and welfare can take he kid away from both of you.
Good luck