badmammy, I can understand why you are finding it so hard to decide what to do. It feels like you are sacrificing your eldest DS (who is so much more vulnerable because of his SN) for the sake of your younger children.
But, IMO, if you contact someone now, you are protecting all of your children. CAMHS are there to deal with your DS's mental wellbeing. But, we know that sexual activity between people in a position of (relative) power and people in a position of (relative) weakness is not always about 'nice feelings' or being 'enamoured'. It is quite possible that your DS either:
a) Has had some hormonal changes which he has explored inappropriately
or
b) Found these things 'fun'
or
c) Knows they are wrong, but (as you say) doesn't comprehend the reason they are wrong, and what it all means
or
d) Sadly, could just be developing a 'taste' for sexual activity with children (paedophilia).
That will be important for CAMHS to try and unpick.
However, and I speak as a mother of a child with complex SN, you can't protect him from 'himself'. Regardless of the reason, your children have been assaulted. It doesn't matter if that has happened because your DS 'innocently' explored feelings in an appropriate manner, or if he is developing a like for young children. The fact is, they have had to do things that they wouldn't have chosen to do if they hadn't been persuaded by your eldest DS.
Social Services are there both for you, your DS and your younger children. They can help you to do the hardest thing. I know they have a bad reputation, at times, but they really are there to safeguard your whole family.
I know it is nothing compared to your situation, but my (now) 6 year old has SN. She goes to Special school. She gets quite frustrated, and as a result, was hitting and kicking her sisters (then 2 and 3). It got to the point that I couldn't keep them all safe at the same time.
DD1 wasn't safe to be left alone at all, and couldn't be put in her bedroom. We only had a lounge and kitchen downstairs. The younger 2 girls were too young to leave alone upstairs, so had to be in the lounge.
DD1 would ping-pong from one to the other, hitting, kicking, whatever. She could see that I couldn't keep them all safe. If I was with her, the others wanted to see me, and as soon as they came close, she would swipe them. If I was with them, she would kick the one I couldn't reach. It was impossible.
Social Services assessed (they had been out when she was 3.5 and refused help). They could see that DD1 was quite violent towards me (at the climax of the SW visit, DD1 was stood on my lap hitting me - as soon as I held one hand, she swiped with the other - and then she declared "THIS IS FUN). She was giddy and completely unreachable.
Long story short, DD1 goes to after school club twice per week, I have social services funded agency care 2 nights per week (an extra pair of hands) and Homestart on the remaining day (the day I actually get a break).
DD1s behaviour is unrecognisable now. She very rarely hits out or kicks. It's because her needs are being met. She wasn't being malicious - she was reacting to her environment, and her hitting and kicking was saying "I can't cope with this".