Hi OP
I'm an ex-social worker and now work in child protection, so I hope what I say has some value.
Firstly, I hope you are you, I appreciate you must be reeling, it's a really awful situation.
I notice you stated he has other disabilities which mean he is not a risk, despite showing signs of being a sexual predator. I'm wondering if there additional disabilities relate to sexual function? Regardless, this will make little to no difference in the assessments Social Services undertake.
From my experience, SS will not accept your eldest remaining in the family home. He will need to find somewhere else. If you refuse, it is almost certain your younger children will be removed from your care. If he truly has been trying to groom children for sex, and social services decided he wasn't a risk and then something happened to your children, could you imagine the backlash? There is no chance they will allow this. You will also be put on some form of a support plan to ensure you do not have your son around your children.
This will not be a short-term matter. Your son will not be allowed to mix with children alone again. The fact you state he doesn't understand what he's done, almost make him more of a risk than some with capacity. How do you explain to someone that what they have done if very wrong if they have no understanding of what is meant by wrong? It's not possible.
This is a very serious matter and if you try and minimise it, SS will see you as a risk to your younger children. I'm sorry, but it's true. You have to show you can protect them and are willing to.
As others have said, I'd also be prepared for this not being the first time and/or instances of child abuse being downloaded. If this is true, it could well be a jail sentence your son is looking at.