The way to ask him is:
Bring the list of symptoms with check marks by the ones you have observed. You want to help him. You will listen to what he says.
Start in a friendly way, expressing an interest in him.
Bring up the subject of CSA, very gently mentioning that you know there is such a thing and that lots of guys get into something like that. Perhaps mention that you have been reading on this website.
Mention that many guys on this website kept a secret when they were boys because they were ashamed or threatened or both, but that they now realize that they had been manipulated and that if they had shared the information it would have prevented much pain.
If he shuts it off don't consider it a failure, but the next you see him quietly mention that you are willing to listen.
Usually when the secrecy bubble bursts he will be well on his way.