It is incredibly upsetting and frustrating when you are bending over backwards to facilitate contact between father and child and the father keeps messing you both around. I can understand how people get to the point where they say enough is enough, no contact. Unfortunately it turns into a power battle between the parents - an extension of their soured relationship - and the child can end up being treated as the least important person, even if neither parent intended that to happen.
I can put myself in your position and see that you have little trust in your son's father. It might be the case that, at the time, he messed you and your son around in order to retaliate for your going ahead with the pregnancy. I think some men find it very difficult that they have no choice in the matter as to whether their (ex-)partner terminates a pregnancy or has the child. And they behave badly to try and assert their will over the situation. I don't know if this is true in your and your ex's case or not. It's not nice for either of you but it happens.
He suggested mediation at the time. It could have just been another stalling tactic, to annoy you, or it could have been a genuine request because he felt that you both needed to involve a third party in order to stabilise your relationship and the matter of the contact visits and maintenance. If things got heated between you (which I assume they did), many men just can't or won't handle an angry woman scorned. They refuse to deal with all the emotions. He may also have needed time to adjust to what had happened. You had no choice as you were pregnant and then had your son, you had to get on with it. In my experience, and at the risk of being sexist and patronising to men, yet again, some men tend to take a lot longer to get their heads around a situation like this.
He already has children with someone else. Presumably he already has maintenance to pay to them and makes time to see them. Sad to say it but, from his point of view, your child further 'complicates' matters for him and his lack of control was difficult for him. It is easier for him to just 'opt out'. We may find this appalling and incomprehensible but not everyone feels the same way.
It is fine to say that everyone should behave well and parents have a duty to their children. This is how it should be but sometimes isn't when the circumstances are difficult. You and he had a conflict of interest. I am in a somewhat similar situation so I do understand.
At the end of the day, as others have said, you must be honest with your son that he has a father. You want to protect him and yourself but this issue won't go away and needs to be revisited. Now is a good time because a couple of years have elapsed, things should have calmed down, he has had enough time to get his head around the situation. It is important that you do the right thing because if you don't, it will weigh on you and you will be stuck with the angry feelings indefinitely and the burden of worrying whether you did the right thing. If you behave immaculately, you will not have any guilt about your part in this and you will feel at peace that you did everything that you could to do the best for your son.
So I would contact him again and suggest mediation. Be patient for a while (say 6 months) but then, if he keeps delaying or messing you around again repeatedly or refuses, then you will know that you were right and it is his problem. Your son can try and make contact with him when he is older if he wants to.
But, if he responds, you may find yourself on the way to a better situation where there is contact between father and son and everybody is happier. It is very hard to open up again and to try to trust when you have been treated badly but the only way to try to improve things is to be patient and give a little. That way you put the onus on him. While you are being angry and refusing contact, he can blame it all on you but if you take away all his excuses, call his bluff, it can become clear whether he cares about his son or not. This is the point you need to get to. Your son need not get hurt in this process and you don't need to tell him anything until it becomes clear whether there is going to be contact or not.
There is no point expecting the father to see his son every weekend and pay maintenance and be the perfect father and ex-partner. He should be this but life isn't perfect and people aren't either, some a lot worse than others. You can't change this, you have to settle for what you can get, put your hurt aside and make it just about your son and his father. If he sees your son once a month and perhaps pays a bit of maintenance, that would be a result. You will always know that you tried your best. If things never improve and the father of your child ends up messing him around, at least your child will learn this for himself rather than fantasising about a mystery father.
Sorry, very long-winded.