My 5yr son had the same problem I think it was thrush I treated with cansesten cream and he stopped itching within a few days
I am wondering if he maybe allergic to the material the onesie or diaper is made of or a cream or bath gel ...6 months is a while I would say something else is going on .what else is next to his skin any vest , have you changed the laundry detergent?
My son is 2 and he started daycare back in June of 2011 and he seemed to get the itch around August of 2010. He gets really red and scratches until he is raw and bleeding. I have been to the Dr's several times and she finally gave him lotrimin cream and an oral antibiotic for a yeast infection however the dosage in done and the redness has somewhat disaPPEARED BUT he is still itching. I have to put his onesie over his pants so he cant get in his diaper. I dont know what to do. This has been going on for 6 mos. now.
My son used to get this kind of itch last year (he was 4yrs old) but he is circumcised. His penis was getting bigger that was all. To keep him from scratching I had him come and tell me every time it was itching so I could give him lotion to rub on (it's an all day task but it did work). Just think of the itch we get when our breast grows. Always have lotion available even for when he is in school. Hope I was helpful.