I have heard of adults doing this as a sexual fetish. Some apparently want the entire baby experience and for some it is more about the physical sensation of the diaper and not about being a 'baby'. I think I would probably take a hard look at what he is watching online and at friends' houses to see if he is watching some sort of porn that focuses on this. Yes, there IS porn about this out there, at least according to CSI (old episode).
I would try to make him feel able to discuss sex and sexuality in an open, honest and appropriate way with you and his father. It seems you are in the early stages of his diagnosis but of course the issues themselves are not new to you. I do think this may be largely due to physical sensation and likely the ability to not have to stop watching/reading porn when he has to use the restroom. He can just go in the diaper and not have to deal with the mess of laundry and explaining the mess to you. If he tried it for this reason and has done it for a period of time, it may have become entangled with his fantasies. I don't know if you can separate that from his fantasies, or if that is a good idea or a bad idea. You have to discuss this with his psychiatrist and other doctors (whomever is most appropriate to speak to, not all of the various docs/therapists, of course).
Just because a person is n the autism spectrum does not mean that they don't go through puberty and all of the issues that deal with becoming an adult. It especially does not mean that sexual urges do not exist. It is IMPORTANT to have open, honest, and frequent discussions of sexuality, sex and the societal rules regarding this whole thorny thicket of issues. Kids with autism do NOT understand the unwritten rules that most of us learn by watching and listening to others. They figure out the rules that make sense to them, and they generally are pretty rigid about these rules. For example, if a young person with autism hears women referred to as "bi$#&$" once they reach puberty, the autistic person may believe that it is the rule to call all women of that age by that word. Then they do this in front of the principal or grandma and everyone totally freaks out and the autitic person has no clue as to what they are upset about. They have heard 'everyone' say this, so why are they in trouble? My parents solved this with my bro by defining some words as 'out back behind the barn with the guys' and others as 'acceptable vocabulary'. "Barn words' were ONLY said when no adults could overhear and that one annoying kid who tattles constantly isn't around. The rest of the time, those words get you into trouble.
The reason this rule worked is because clearly those words ARE used in the world, and it is hard to explain every situation. If the rule doesn't make sense to the autistic person, they will find their own rule and that can make situations VERY complex. This was an easy rule of thumb that we could use to make sense of the world.
Sexuality is complex and helping an autistic person learn to handle his sexuality is a real challenge for parents. Ignoring the issue will NOT help. Your difficult child is going to explore this part of himself and it is up to you to help him adjust his impulses to appropriate behaviors. If he isn't hurting himself or others, and the psychiatrist is in agreement, maybe he can earn the money to buy the diapers and he can go buy them himself. Of course stealing them isn't appropriate so this has to be stopped (if that is how he is getting them). I would also make sure he isn't watching porn online or at other peoples' homes because that will give him some really inaccurate expectations that reality simply cannot meet. That results in a whole ton of other problem that are tough to deal with. (Not to mention whatever your religious/moral stance on the issue of porn is)
If it was my child, I would wonder if it is a certain sensation that is desired or if it is the idea of being taken care of like a baby. If it is a specific sensation, there may be a more socially acceptable way to provide the sensation (the squishy feeling of the diaper after it is used, the pressure of the diaper, who knows?) than by using a diaper.
I do think that the guidance of a good psychiatrist (psychiatrist) and therapist (psychologist) can be very helpful. Also an occupational therapist may be able to help provide more socially acceptable ways to manage whatever sensory needs the diaper is taking care of. Sensory issues are a BIG part of the problems with autism, and regardless of age, an appropriate sensory diet can be incredibly helpful for a person with any degree of autism.
I hope this helps!