If you avoid the "I'll tell you something but you can't say anything to anyone" trap, you can say to him (if he does disclose abuse) that the abuser wants him to keep it secret. He may have tried to make it go away or forget about it but it probably keeps coming back. He can't fix this by himself because if he could have, he would have.
You can reassure him that you have some contacts with men who have been abused and there are resources here to help him and protect him if that is still an issue.
You have taken that first step of disclosing to someone and a community of people who understand and can give him some support and advice, if wanted.
If he tells you, there is a reason. If you somehow pull it out of him, he still could have denied but even if he admits reluctantly, it is a sign that he knows deep down that he couldn't fix this by himself.
Blissfully retired after 35 years treating sexual abuse