Take heart. As the saying goes, "This too shall pass", and before you know it he will be on to the next uber-annoying fad. My oldest (ten now) never had a costume thing, but more 'latest obsession' things have passed through my house than I can count over the years. First it was Blues Clues - I STILL know all the songs by heart (as I found out the other day when my three year old twins were introduced to Blue by big bubba....thanks, Jack). Then came The Wiggles (yay)...Bob the Builder.....Pokemon....Danny Phantom....Ben 10...Now it's iCarly (because he has a HUGE crush on Miranda Cosgrove). And each time he moves on to a new thing, the old thing is suddenly "for babies" or "sooooooo old".
I remember Jack was 3, he used to go around singing the Blues Clues song everywhere we went and wouldn't go anywhere without his Blue doll. People would give me 'the look' as he sang at the top of his voice, but I didn't let it bother me....and now I'm so glad I didn't, because the memory of his tiny little voice and the happiness in his eyes makes me smile as he rolls his eyes and tries to escape when I want a hug from my 'little boy'. These are the times you will cherish because before you can blink, your baby will be a sulky, know-it-all, teenager-in-training like mine!