Hi all :)
First of all, you are doing everything right as you have decided to hear him out and not judge untill you have further understanding of the situatuion. That is the way to go. Jumping to conclusions or decissions in matters like this can cause pretty awkward and in the long run damaging situatuons. Your son share these feelings with many, myself included. I have always felt that diapers give me a sensation of deep relaxation, comfort and security. As i matured and entered puberty, the wish to wear diapers evolved into a sexual fetish. Wearing diapers triggers in me a feeling of happiness and joy - the advantage of a fetish. In this way diapers has always been a positive thing. There is though, a downside. Not with diapers in particular but with the general feelings of guilt and shame that comes with not being "normal" or built to fit in with the general crowd. This is a fight every one must fight sometime in their life, but for someone with a different sexual orientation or a fetish that unlike mainstream leather and latex fetishes is not generarly accepted in the society, the fight can become a very long and hard one. Especialy with close family members judging you or rejecting what you do because it is concidered wrong for one reason or another. There is nothing wrong with anything you choose to do, as long as it is safe, sane and consesual. As long as you are not hurting yourself or others in the process - what can possibly be wrong in doing something that makes you feel good?
Anything like this can become an obsession, and ofcause nothing good will come from that. A good way to induce a obsession is being denied your feelings and desires and attatching to them the feelings of shame and guilt from being seen as weird or looked at with disgust. Im not saying that this is how you view what he is doing, but I know from personal experience that many people do, and unfortunatley its a harmful way to any relationship, be it within the family or outside.
I talked to my parents when I was 17 because I could not stand the hiding and sneaking, and the fear of being discovered and of the following rection. Plain fear.
And because I needed my diapers so bad I knew I could not live without them. I told them as honnestly as I could of my feelings, and it was the best decission i have done. We talked alot - they had many questions and I did my best to answer them. Once they had realized that it had nothing to do with pedophilia or actual children, and that this was a part of me that I knew would not go away they accepted it and was very supportive. I wore diapers around the house a lot from that point, but made sure to be discrete about it. I have always thought it to be important not to push your personal feelings, thoughts and beliefs upon others who do not share mine. So I bought my own diapers, made sure to use and discard them without it being too obvious or "in your face" kind of way. Im sure they knew from time to time what I was up to, but never mentioned it as far as I can remember.
If you decide to let him go along, a suggestion might be that you make up rules as to when and what is OK with you, because you must feel comfortable with the arrangement too. Have a honnest, straight forward talk. Then make up your minds and go along what you both can agree on.
This is your sons "thing" and it becomes a problem and an issue when it feels like one to him.or if he can not keep a apropriate distance to his fetish and becomes obsessed or starts pusing it at others.
Good luck both of you, and best of luck!