OP, You say that all you ever wanted was for your XP to have a relationship with your DS. By being accommodating, as you have been until you rang the CSA, has it encouraged him to build a relationship with his DS?
No.
Personally, my feeling is that he'll be more inclined to try to build a relationship with DS if you make him take responsibility and pay maintenance, so I would forge ahead with this.
Your XP had a choice. See his DS, build a relationship with him, and you would have been more relaxed about maintenance. He chose not to build a relationship with DS, and now you want the bare minimum...maintenance. He cannot complain. He has been offered a choice very few father's are offered. If he truly could not afford to pay (and clearly he can afford it, probably loose change to him) he should have built up a relationship with DS.
Anyway, enough of my ramblings lol.
My guess is once he is made to pay, he will be much more likely to see and build a relationship with DS, because lets face it, who wants to pay to support a person they never see, hear from, or interact with??
For your XP, I'd imagine paying and not seeing DS would be akin to watching a little bit of his money vanish every month for the grand total of nothing in return, and from what you've said, your XP doesn't sound like the charitable type. He will probably want to receive something back for his money, and that something, the only thing on offer, is a relationship with his DS.
If you still decide to back down, you do realise he will lose all respect for you and DS, and will probably be silently cheering at his good fortune to have had a child with someone who is going to play entirely by his rules all of the time.
Your DS deserves this money. He deserves to be acknowledged by your XP, even if it is only financially.
So sad for your DS being denied a relationship by your XP. Please don't take away the last remaining pennies your DS is entitled to, because you feel sorry for your XP. He doesn't deserve your empathy.