One of the great perennial topics, and one I always answer, even if I've done so many times.
For me, it was a pair of little girl's panties, when I was about 4 or maybe 5, and my mom put me in them! We were at my aunt's house, and my girl cousin and I were playing in the backyard... I don't recall exactly what happened, but for some reason I got dirty enough that I had to change all my clothes. This was a day trip, so I had no extra clothes, so my aunt graciously provided some of my cousin's. I don't remember thinking anything special about the top and shorts, but the panties (sturdy, plain white cotton early-70s briefs) fascinated me...they felt so funny 'cause they came up way high on me, and they had little ruffles around the legs. I remember that I pulled them up so high that the waistband showed, and my aunts teased me about it.
If you want to give a boy a fetish, have his mom put him in something "different" at an early age, make sure he gets attention, and then wait 'til puberty...sure enough, I've been obsessed with wearing panties ever since--particularly little girl panties. I was well into my teens before I "graduated" to women's, long after the girls my age would have, and I still like a nice pair of sturdy white cotton briefs.