SK1ER - I have seen your other thread and posted on it too.
I don't think you have said why your ds's father was denied PR. It may be relevant..?
The court process has changed and things moved forward. It is now considered better for both parents to be involved when before it was thought better that one parent be totally responsible. Even unmarried parents are automatically given PR if they are named on a birth certificate now, and can get it relatively easily if not. Unless there are good reasons why PR should be denied he could well be given PR this time. Don't assume he won't be or that he has lied by omitting the previous court case in order to achieve it.
He must know you would bring it up if he lied about it and be able to prove it. He has perhaps forgotten or not believed it to be relevant for some reason... Why?
And while you may be right that there is possibly a link between your CSA application and his PR request it may not simply be tit for tat. It may well be that he is thinking, well if I'm going to pay for my child then I sure as heck and going to be involved and get that PR now.
Going through the court process isn't easy and it isn't cheap. Kess dedictaed parents may well give up when they realise the time frame and cost involved.
Bear in mind that if he has a job then he is unlikely to get legal aid. If he doesn't have a job then how come he is having to pay more via the CSA which has triggered this (as you feel)? Also if he is on legal aid he'd be required to attempt mediation and you say he hasn't suggested it. SOmething that leads me to think maybe he isn't legal aid funded. Are you sure he has legal aid and isn't funding this decision himself?
People do and can change. They can grow up and it can take something like you persuing him via the CSA for him to sit up and say 'you know what, I am a dad and I'm going to be a dad before it's too late'. Especially when his son is saying I want to know my dad and see him.
I don't deny that there is also a possibility that he is simply doing this to
get at you. But court and CAFCASS are well used to seeing this and hopefully would spot it if it is really his main reason for persuing PR and contact (and believe me I am no big fan of CAFCASS or their abilities but even they can spot someone who isn't genuine and child centred in their choices).
Your ds is old enough to have a say in seeing his dad. I believe you said he wants to see his dad but doesn't want him to have PR. Well I'm sorry but he has responsibilities to his son, whether they are officially recongised or not. Ok so perhaps he hasn't lived up to them as you would have like up till now. But you can't deny him his responsibilities.
I feel that you do both need to go to mediation. Stop asking why he hasn't suggested it and suggest it yourself. You will both be angry and probably not achieve very much to start with. But if he (and you) are serious about finding a solution in the best interest of your ds you will go back and try again once you get past some of that past pain and bitterness.
If he can't/won't then maybe that will show you are right to doubt him. But don't assume things are as they were before or that he hasn't changed. Are you still the same person you were ten years ago?
Even so it still doesn't stop the fact that your ds is saying he wants to see his dad.
I would be highly surprised if a young teenager has any concept of the phrase PR and what it means unless they had been told about it. Have you been telling your ds about it and what it means and has he then decided he doesn't want his dad to have it? Be careful. You will look like you are trying to dictate this decision if he is too well informed about what it means. I don't know any child, teenager or not, who says 'I want to see my dad but I don't want him having PR' 
I understand you have concerns and are worried. But why? What is behind his? And are you not willing to accept that your ex (his dad) may have changed? He has after all got a hob now, surely that's proof of some change in his life...