I'll try to make this as brief as possible, but it may end up fairly long.
DS is nearly 8. Ex and I had only been together 1.5 months when I fell pregnant and had actually already split up when I found out. We got back together for the sake of the baby but I ended it a couple of months later as we really weren't suited to each other at all.
During pregnancy, ex made promises he never kept. First 2 years of DS's life, he broke more promises, sent me letters and emails saying he needed space and didn't want to be bothered by us, interspersed with letters and emails asking why I 'wasn't letting him see DS'. He spread lots of lies, many of them vicious, about me, and said truly nasty things about my mother to his friends and family, because she told him to stop being so immature and to get a decent job so he can support his child - the one he claimed to desperately want. To be completely fair, I wasn't particularly accommodating myself as I was suffering with PND and his behaviour during the pregnancy had really angered me.
When DS was 2, I said enough was enough and that I wasn't going to have a young child put through the emotional wringer every few weeks with his 'Dad' letting him down, lying, changing his mind and claiming that I was spending the CSA money on going out. Because £5 a week buys a good night out 
! During those first 2 years, there were periods of months at a time where I didn't hear a peep from him. Also, he assaulted his gf in front of her son during that time, although it was pushing rather than hitting.
Ex sent emails denying paternity and saying he would take legal action to see his son. I said to go ahead and get tested and if he actually went ahead and took me to court then that would be fine because he'd at least be showing that he cared about our son.
For the next few years I got emails maybe once a month, often less. Each email coincided with a CSA payment arriving (once they'd caught up with him again as he kept changing jobs to avoid them). Each one said 'I've paid my money, I have a right to see my son'. Not a single one said he actually wanted to see him, and none of them asked after his wellbeing or schooling or even for photos. He just kept going on about how he has a right to see him as he's paying maintenance.
The emails stopped a couple of years ago (still paying maintenance faithfully now though). I have since changed my email address so he doesn't have any way to contact me. I didn't think about that before I changed it, doh!
Now, part of me is thinking I should get in contact and arrange for him to meet with DS, especially as the ex now has another child with his current fiance, so DS has a half-brother. Another part of me thinks that if he really wanted to get in touch, he could easily apply for a contact order or a PR order (he's not on the birth certificate). Or he could find me on the electoral roll via 192 for just a few quid.
This is so long, I'm really sorry. But I wanted to give a fuller picture. What do you think I should do? I'm so torn - I hate to see my DS to be constantly let down and hurt, but then all of us hate to see that!