I would say that your son has developed a fetish for wearing diapers.There are many pages that will help you gain some understanding of your sons desire.Dpf is one that will offer some help. You can also type infantilism into your browser. The most important thing is to try and except that your son does have this desire and that it may have been going on longer than you realiser and will be quite deep seated. You will not be able to cure him of these feelings and if you try you will just make him drive his feelings underground to arise at a later date with him feeling guilty and lead a life of secracy.