OK ready for the flaming
OP have you ever, even for a second, suspected he was a danger to your son?
2000 images are not downloaded in a day. He was doing this while he was seeing his son (I don't mean while your son was in the room I mean over a period of years),
I know this is not a victimless crime. I know the conviction is for a serious offence but surely there are two questions.
Is the father a danger to his son?
Is there any reason to stop contact if he isn't? By that I mean do they have a good relationship? Would your son miss out from breaking contact?
Before the flaming, a conviction does not necessarily mean he is going to hurt his son. Yesterday we had a long thread deleted which was basically about a mother leaving her daughter with mother's boyfriend and the child being murdered. As far as I know that was that man's first offence, no convictions up until then but there were warning signs.
I agree with all the comments about talking to SS and the police officer in charge of 'managing' your ex.
I can't understand why they would allow something like this? Because the court issues the punishment in this country. Because your son may not be deemed to be at risk.